Founder of SDP, which went on to form the Liberal Democrats with the Liberal party, launches No to AV, Yes to PR campaign in letter to the Guardian

Hopes of replacing the first-past-the-post voting system with the alternative vote (AV) have received a blow as Lord Owen, one of the founders of the Social Democratic party that went on to form the Liberal Democrats with the Liberal party, helped found a group opposed to AV in favour of "real reform".

The "No to AV, Yes to PR" campaign was formally announced on Friday in the Guardian's letters page, with the support of Owen, his fellow crossbench peer Lord Skidelsky, and other eminent figures including the Bishop of Blackburn, Nicholas Reade.

It is designed to attract those disaffected by the decision of the longstanding supporters of proportional representation – such as the Liberal Democrats, Joseph Rowntree Reform Trust and the Electoral Reform Society – to back the Yes to AV campaign.

In a separate intervention, a group of leading historians sent a letter to the Times (paywall) in which they argued that a switch to AV would undermine the principle that each person's vote is equal.

The criticisms of the AV referendum – a key demand made by the Lib Dems as part of the coalition deal – comes as Nick Clegg prepares to face party activists in Sheffield for their spring conference.

Before the general election, Clegg had described AV as a "miserable little compromise" because it stops short of full proportional representation, which is based on the share of the vote gained by each party.

Clegg is now making the case that AV will ensure more people's voices will be heard, while David Cameron is pressing the Conservative line for a retention of the status quo.

Under AV, voters rank candidates in order of preference, with losing politicians in each constituency eliminated and the preferences redistributed until one has more than 50%.

Owen and others argue that many will share their belief that a "third choice", of a proportional voting system, should have been included in the forthcoming referendum on voting reform on 5 May, and that clinging to AV as the first step towards a fairer system of proportional representation is misguided.

"This is particularly so since proportional representation has been at the core of election campaigns over at least 30 years by the Liberal party, the SDP, the Green party and, up to late 2009, the Liberal Democrats," they wrote.

The authors said they would "reluctantly vote" against AV.

"We recognise that some of those strongly committed to proportional representation genuinely believe that the alternative vote is an incremental step to the fairer system of proportional representation. But we do not accept that the electoral voting system can be subject to repeated reform. Once changed, a new voting system has to be tested over a substantial period of time – otherwise it will destabilise our political system and encourage cynical attempts to change the system for reasons of partisan advantage."

Clegg said in an Independent interview on Friday that he had joked to Cameron that he was talking "complete bilge" when he defended the first-past-the-post system at prime minister's questions on Wednesday.

But Cameron's position is shared by senior academics and historians, who drew on the lessons of history to warn that AV would "for the first time since 1928" present the possibility that "one person's casting ballot will be given greater weight than another".

In their letter to the Times they wrote: "For the first time in centuries, we face the unfair idea that one citizen's vote might be worth six times that of another. It will be a tragic consequence if those votes belong to supporters of extremist and non-serious parties."

They argued that the cause for reform could not have the "fundamentally fair and historic principle of majority voting cast aside".

"Twice in our past the nation has rejected any threat to the principle of one citizen, one vote. The last time, in 1931, Winston Churchill stood against the introduction of an alternative vote system. As he argued, AV would mean that elections would be determined by 'the most worthless votes given for the most worthless candidates'. He understood that it was simply too great a risk to take."