More than 120 Labour MPs – a third of the parliamentary party – are preparing to quit Westminster at the next general election in the biggest clear-out of the parliamentary "old guard" for generations, according to senior party figures.

Last night, the party released figures showing 63 Labour MPs have already informed Gordon Brown they are going.

Information passed on to party whips suggests this total will rise to some 93 by mid-October, and could then climb by at least another 20-30 in the run-up to a general election, which is expected next spring.

Party insiders say MPs' experiences when confronting voters incensed by the expenses' scandal has added to a sense of disillusion about the job as Labour heads for what many now believe is inevitable, thumping defeat at the next general election.

Many Labour MPs are concerned that their earnings will be pegged back severely if pressure mounts over coming months to stop them taking second jobs in order to supplement their £64,766 salaries.

One senior Labour figure said: "The total will go well over 100, probably to 120. After 12 years in power, Labour MPs do not want to be in opposition for a decade bound by rules that prevent them realising their earning potential." Andrew MacKinlay, a Labour backbencher who recently announced he was quitting, said: "A lot more will go, I am sure. It will probably reach 120. That should be the high-water mark."

Labour MP Barry Sheerman told a private meeting of backbench MPs last month that the party could face mass resignations if discontent with Brown's leadership deepened. "If we are not careful, a number of MPs could leave their seats in the coming weeks and months and cause byelections because they are fed up with what is going on – and then we would be in real trouble."

But most Labour MPs say they and their colleagues will avoid going before the election so that they can take advantage of tax-free severance terms that guarantee MPs between six months' and a year's pay if they leave parliament at a general election, whether voluntarily or not. "This is under review, so everyone is thinking: now or never. If you go before an election, you get nothing," said a source.

Increasingly MPs believe that Brown may announce further restrictions on second jobs following a report into MPs' expenses by Sir Christopher Kelly, due to be published in the autumn. One senior Labour source said he believed there would be a manifesto commitment to ban second jobs. Harriet Harman, Labour's deputy leader, is known to favour making the job of MPs "full-time".

Last week, Eric Pickles, the Tory chairman, said he expected about another 17 Conservative MPs to announce they are stepping down, in addition to 26 who have already done so. Six Lib Dems have so far announced they will leave parliament. At the next election, parliamentary authorities expect about 200 to quit.