Party leader confirms he will vote for increase in student tuition fees but accepts backbenchers may not 'walk through the fire'

Nick Clegg tonight resigned himself to a Liberal Democrat split in the vote on planned increases in tuition fees, as he told his MPs he and other wavering ministers in his party would vote in favour of the plans but accepted many of them would not "walk through the fire" with him.

After a parliamentary party meeting in Westminster, Clegg appeared to have partially ameliorated the split by persuading ministers who had once indicated they could not vote in favour of the planned increase. Aides to the deputy PM said there would be at least 24 of the party's 57 MPs voting in favour. But abstentions and votes against the measures leave Clegg facing a three-way threat in his party.

The new line will test the ministerial code of conduct. Today the party high command agreed parliamentary private secretaries – ministers' aides – could abstain on the plan. But the new code, drawn up by David Cameron in May decrees that a PPS must toe the government line or face resignation.

The meeting with MPs was designed to iron out the party line ahead of the vote on Thursday, seen as the most difficult parliamentary test since the formation of the coalition. The Lib Dems signed up to a pledge not to increase tuition fees but the coalition government now proposes lifting the cap to up to £9,000 a year.

The coalition agreement allows Lib Dem MPs to abstain but not vote against the proposals, something many Lib Dems have felt they must do to truly be faithful to the promise. In a bid for unity, last week the business secretary Vince Cable floated the possibility that he would abstain, despite admitting that he was in favour of his own proposals, but that gesture was soon withdrawn. Whips are trying to avoid the energy secretary, Chris Huhne, having to be recalled from the climate change summit in Cancún.

In three-hour meeting Clegg told his colleagues they had dealt with intense pressure on the vote with "great dignity", and that he was very proud there had been no animosity. He praised Cable for coming up with the policy, remarking it was striking that despite the financial circumstances, under the new arrangements no student will have to pay anything up front.

But he reminded them that "to govern is to choose" and that the Lib Dems had chosen to concentrate on investment in early years funding rather than higher education – a commitment to increases the life chances of younger children that, he reminded them, was emblazoned on the front of his party's manifesto going into the last election.

Clegg admitted his attempt to create party unity had not worked and people would not now "walk through the fire" collectively – this was not now possible.

Earlier in the day, darling of the party and former Labour education secretary Lady Williams said those Lib Dems who had signed the National Union of Students' pledge, now causing so many of them such heartache, had been unwise to do so. She said: "They did it in good faith but had a very, very sharp awakening when they couldn't [implement the policy]."

Newly elected Tory MPs Lee Scott and Andrew Percy said they could not vote in favour of the plan, joining former minister David Davis in a small Conservative rebellion. Scott is parliamentary aide to the transport secretary, Philip Hammond.

The shadow business secretary, John Denham, made a final appeal to MPs to either delay the vote or reject the increase, saying the proposed £9,000 cap was nothing to do with the deficit, but purely due to a disproportionately large cut to the university teaching budget.

In a letter to all MPs, he wrote: "In a spending review with an average cut of 11%, the government has chosen to remove 80% from university teaching budgets. Even within the parameters of their approach to deficit reduction, the government could have proposed fee increases of hundreds of pounds, not up to £6,000."

"This fee increase is therefore not 'unavoidable' – it is a political choice the government has made. These proposals amount to a rejection of the longstanding recognition of our collective responsibility for higher education."