There has been cross-party condemnation of a "lack of transparency" by Cornwall Council after the local authority publicly released just 17 of 1,700 pages of emails between the council's leader, chief executive, and other senior officials discussing the failed bid for a Mayor of Cornwall.

Independent councillor Tim Dwelly submitted a Freedom of Information Act (FOI) request to view all emails sent to or from the council's chief executive Kate Kennally and its Conservative leader Cllr Linda Taylor mentioning the word "referendum" to see how the Mayor referendum issue had been discussed and considered at a senior level. He also requested all emails on the subject sent to or from outside bodies, MPs, civil servants and Cornwall Association of Local Councils.

He has accused the council of thinking that "public interest is best served by withholding huge amounts of material" after only 17 pages of emails were released after an initial 1,700 pages were found. Cllr Dwelly was happy for the council to "reduce scope" from the initial search to 562 pages of "focused information" but was shocked when he eventually received such a small amount of correspondence.

He said: "That means they are withholding a whopping 97% of emails discussing a mayor referendum." Graham Webster, who led the Let Cornwall Decide campaign for a public referendum, said "it stinks", while other councillors said the council was "keeping secrets" and believed a perceived lack of transparency damaged its reputation.

The matter is now being investigated by the Information Commissioner's Office (ICO), a non-departmental public body which reports directly to Parliament, following an appeal by Cllr Dwelly.

Linda Taylor dropped plans in April to seek an enhanced devolution deal for Cornwall which would require a directly elected mayor. The announcement came a week after results of a public consultation on the issue were published which showed that 69% of people were opposed to the devolution deal and a Mayor for Cornwall. Under the plans Cornwall Council was seeking a Level 3 deal with the government which would have devolved some powers to the council and provided £390million of additional funding. The council is now seeking a lesser Level 2 deal which does not require the need for a mayor.

The council department which handles FOI requests told Cllr Dwelly that it had "concluded that the public interest test favours non-disclosure because although disclosure of this information would increase access to information held by the authority and allow scrutiny of the public authority's decisions it would also prejudice the free and frank exchange of advice and views for the purpose of deliberation.

"In conclusion, in all circumstances of this case we are of the opinion that the balance of public interest favours non-disclosure of this information at this time."

It also stated that there was a cost impact on extracting the relevant information. David Coulthard, the council's strategic business delivery manager, told Cllr Dwelly: "It's worth noting that Section 12(1) of the FOI Act is a provision which allows a public authority to refuse to comply with a request for information where the cost of compliance is estimated to exceed a set limit known as the appropriate limit.

"The appropriate limit has been specified in regulations and for local Government it is set at £450. This represents the estimated cost of one person spending approximately 2 1⁄2 working days (18 hours) in determining whether the council holds the information, and locating, retrieving and extracting the information. In terms of this specific case, it took the council in excess of 80 hours to extract and review the information that was initially captured as 'potentially within scope'."

Following his request for more information, Cllr Dwelly wrote to councillors across all parties stating: "You will note the council's letter giving reasons why they have withheld most of the emails on a mayor referendum from the public. I am appealing this. In my experience as a cabinet member I never once received an email from any senior officer that I felt was 'too secret for FOI'. The letter actually says, and I quote, 'public interest favours non-disclosure'.

"I hope your eyebrows will rise as high as mine did at this position on a matter that dominated a whole year at Cornwall Council."

He told us: "The people of Cornwall will not be impressed that the current Conservative council won't let them see what senior officials and the council leader were saying about who should run Cornwall. They are keeping secret an astonishing 97% of the emails they found. For this reason I have asked the Information Commissioner to review this decision and hopefully insist that all the material is released, which is clearly in the public interest.

"We almost had a Mayor of Cornwall imposed on us in a shabby manner without a public vote. We all have a right to see what the highest paid officer and councillor in Cornwall were saying about this. Why did the council bend over backwards to stop a referendum?"

Politicians representing a range of parties were quick to join Cllr Dwelly in condemning the council.

Conservative councillor John Conway said: "I concur with Tim. The secrecy itself asks more questions. Members [councillors] are responsible for policy, why do senior officers appear to be covering their tracks, did they exceed their authority? We as councillors have always been told that all emails on Cornwall Council computers are subject to FOI. If there is nothing untoward why are the emails being withheld? If you were to FOI my emails on a given subject would the officers be so keen to withhold them?"

Cllr Colin Martin, the leader of the Liberal Democrats group at the council, said: "The change to a directly elected mayor would put far more power in the hands of one politician and any officer who can control that individual would effectively control the whole council. It should be obvious why this may be an attractive prospect for the powerful few, but bad news for the rest of us.

"Did the leader and senior officers discuss ways to manipulate or disregard democratic opposition in order to secure their preferred outcome? One of the core values of Liberal Democracy is that those in positions of authority should be transparently held to account to ensure that they are not misusing their power. When powerful people want to keep secrets, voters have every right to be suspicious."

Independent councillor Julian German added: "It is a disgrace that Cornwall councillors cannot get hold of information. There is no way that councillors should need to go to the Information Commissioner's Office to obtain information about correspondence between the leader and chief executive. Councillors having to do this shows a lack of transparency from the current administration and damages the council's reputation with the public. It's a sad day and I urge the council to hand over the information now."

Perran Moon, Labour's parliamentary candidate for the Camborne, Redruth and Hayle seat, said: "It is frankly deeply concerning that the Conservative-run Cornwall Council is withholding scrutiny of how much time, effort and cost went into 'selling' the failed idea of a Mayor of Cornwall. Healthy, democratically-run councils are those that welcome scrutiny, not shy away from it.

"In relation to this particular Freedom of Information request, the cynic might conclude that Cornwall Council is ashamed of admitting to a massive and costly campaign to foist an unwanted Mayor of Cornwall on us. If Cornwall Council have nothing to hide, they need to stop procrastinating, adhere comprehensively to the Freedom of Information request and release all relevant information, including internal communication, immediately."

Graham Webster, who spearheaded the Let Cornwall Decide campaign, told us: "It says a lot about Cornwall Council that they are not prepared to be open, honest and transparent about these things. This is stuff that should be available in the public arena.

"I know we got the result that we wanted in terms of stopping the mayor situation but for me it was all about having a referendum and the public's right to decide whether we had a mayor or not. I think it's really bad of Cornwall Council that they're not willing to be open and transparent on exactly the exchange of correspondence that went on within the council at the time.

"This is information that should be readily available to everybody – what are they trying to hide? In all honesty, it stinks."

We contacted the council asking why the FOI decision was made to release such a small amount of correspondence. We also asked for a response to the perception that the council is not being transparent and is hiding details from the public.

A spokesperson simply said: "The council is unable to comment on individual cases or requests. However, any FOI request received by the council and subsequent response would be considered and actioned in accordance with appropriate legislation."