A Liberal Democrat grassroots revolt over the party leadership's support for trebling tuition fees emerged tonight as members of the party's policy committee demanded powers to rein in ministerial independence from Lib Dem policy.

Separately, proposals were being put forward by some activists to make it easier for local parties to deselect Lib Dem MPs.

The moves came as David Davis, the Conservative MP and rightwing standard-bearer, announced that he will rebel in the key Commons vote on Thursday by voting against the trebling in tution fees.

Although Davis insisted he was "a rebellion of one," provoked by the damage he said the fees rise will inflict on social mobility, his move prompted coalition fears that a few other rightwing Tories might also break ranks, so reducing, but probably not endangering, the coalition majority in Thursday's Commons vote.

Davis told the Guardian: "The kids being helped are the very, very poor indeed. Free school meals being the bar [for applying for the government's financial support fund plan] means quite a lot of aspirant working class kids will not be helped." He added he was concerned by rising indebtedness. "Kids are already leaving university with high levels of debt before they even go to work. I am worried making this worse will see the next generation significantly set back – unable, for instance, to get a mortgage."

His remarks will severely embarrass the Lib Dem leadership as they claimed the entire tuition fee package as a whole will help social mobility.

Nick Clegg, the Liberal Democrat leader, is to make a desperate final plea to persuade his MPs tomorrow night to retain unity, but that prospect was evaporating tonight. Two junior ministers, the transport minister Norman Baker and the equalities minister Lynn Featherstone are considering whether to abstain, or vote against a move that might require them to resign from the government.

The coalition agreement allows Lib Dem MPs to abstain on tuition fees, but it is not clear how this applies to ministers. At least 13 or 14 backbench MPs, including former party leaders Charles Kennedy and Sir Menzies Campbell, are going to vote against, with Vince Cable, the business secretary, leading a group determined to vote for the measures they support.

Lord Ashdown, the former party leader, today called on the party to back the coalition tuition fees package, but admitted that in the current political climate the public was not listening to Clegg. He added: "Nick could deliver the Sermon on the Mount. They are just not listening."

Clegg's main strategic goal has been to show that coalition politics works, and he will be disturbed that the tuition fees vote is prompting calls for a rethink in the constitutional relationship between the party and ministers.

Gareth Epps, a member of the Lib Dem federal policy committee, warned that "the policy committee elected in November will want to have the opportunity to put out clear policy statements if it feels coalition policies are at odds with the Liberal Democrat principles and policies.

"A more formal mechanism is needed to make ministers realise they need to think again, and this will become more important as the coalition continues and more proposals inevitably emerge that were not covered by the coalition agreement."

Epps said that the party's special conference, called to endorse the coalition agreement in May, was debarred from discussing two motions ordering MPs to stick by their pledge to oppose a rise in fees in line with the manifesto agreement.

Lord Ashdown had yesterday claimed that the special conference had unanimously endorsed the coalition agreement.

Epps said: "It's self-evident now that the failure to tackle this issue at Special Conference has been a significant contributory factor in the difficulties the Party has faced now, and particularly in the disconnect between Ministers and the Party at large.

"The conference organisers did the party a severe disservice by preventing the special party conference from discussing the specific issue of tuition fees. If it had, we might not be in the difficulties we are now and we would be able to rein in ministers as they veer from what was in the manifesto. This is now an issue of trust as much as the policy."

He said two motions demanding Liberal Democrat MPs stick by their election pledge to oppose a rise in tuition fees were not taken by the conference organisers. One motion, designed to refer to the pledge given to the National Union of Students by Liberal Democrat MPs not to support an increase in tuition fees read "no MP can be mandated in matters of conscience, including those matters where they have explicitly pledged a certain course of action to their electorate.'

Another called on the party to reaffiirm its aspiration to scrap tuition fees and to make|" the impact on student debt" the paramount consdieration of any decision.

There are also calls from party activists to change party rules on the deselection of MPs. The current rules, adopted at a time when the Lib Dems wanted to avoid the kind of civil wars that plagued Labour, limit local party ability to deselect MPs with the incumbent given preference. One source said: "Our MPs have to realise they cannot vote for this rise with impunity. It damages not just them but the whole party." In practice redrawing constituency boundaries will make it harder for sitting Lib Dem MPs to claim incumbency.

In a bid to hold the party together cabinet members, including Clegg, have been holding teleconferences with senior party figures including parliamentary candidates to explian his thinking.

More than 135 Liberal Democrat parliamentary candidates have signed a petition urging MPs to oppose the tuition fees increase, and currently the mood is against calling for a special conference, seen as the nuclear option.

'MP' is an impostor



A man interviewed on Radio 4's World at One today, who claimed to be a Liberal Democrat MP, was revealed to be an impostor. Presenter James Robbins thought he was apparent interviewing ministerial aide Mike Crockart, who said he would be prepared to quit over the rise in tuition fees, but the interviewee turned out to be an impostor.

The BBC blamed an incorrect number listed against his name in the corporation's directory of MPs' contact details. All the "usual pre-broadcast questions" had been asked of the man, over the phone, who had "maintained throughout that he was Mr Crockart and appeared credible", the BBC said.

A fake Crockhart was also quoted in today's London Evening Standard, saying he would resign over the issue of tuition fees.

The quotes were picked up by the Press Association, which later issued a correction saying that the quotes attributed to the MP were from an impersonator.

The Lib Dem press office posted on Twitter: "For the record, Lib Dem PPS Mike Crockart was not on Radio 4 resigning earlier – it was an impersonator (he wasn't even Scottish)."

"We are absolutely clueless as to who it was," said a Lib Dem insider. "We figured it out when our head of media was talking to Mike on the phone and the fake Mike Crockart was on the World at One."

The BBC's most famous case of mistaken identity came four years ago: its 24-hour news channel interviewed a man who had been waiting in the BBC's reception., thinkingStaff thought he was computer expert Guy Kewney. Guy Goma gamely bluffed out the interviewer's questions on screen and briefly shot to fame as the "wrong guy".

• This article was amended on 7 December 2010. The quotation from David David that "The kids being helped are the very, very privileged indeed" should have read: "The kids being helped are the very, very poor indeed." This has been corrected.