A former Cabinet minister today blames Gordon Brown's "incompetent and unjust style" for Labour's disastrous by-election defeat in Norwich North.
The party's vote haemorrhaged as the Tories won a convincing victory in the first contest to be forced by the Commons expenses scandal.
Chloe Smith, 27, will arrive at Westminster in the autumn as the country's youngest MP after achieving a swing of 16.5 per cent to capture the previously safe Labour seat by a comfortable majority of 7,348 votes.
The by-election was called after Ian Gibson quit Parliament in protest over being barred by Labour's "star chamber" from standing for the party at the next election.
As Labour recriminations grew over the scale of the defeat, MPs claimed that the Prime Minister's determination to act tough had backfired disastrously.
Writing in The Independent today, Charles Clarke, the former Home Secretary and MP for the neighbouring seat of Norwich South, says that Mr Brown was selective over which Labour MPs were targeted after the expenses storm erupted.
He says: "It was this arbitrary approach which led directly to the by-election as the Prime Minister vilified Ian Gibson, but not on any fair basis. This incompetent and unjust style has deeply damaged democratic politics."
The Labour leadership took swift action against Mr Gibson after it emerged he claimed more than £80,000 in expenses for a flat where his daughter lived rent-free and then sold it to her at a discount price.
His supporters claimed he had been deliberately singled out because of his history of criticism of the leadership and won the backing of many activists in Norwich North when he resigned immediately in protest over his treatment.
Labour mounted a lacklustre campaign in the constituency with little support from senior ministers. It ended with its candidate, Chris Ostrowski, confined to quarantine with swine flu and his wife taking his place at the count yesterday.
When the result was declared, nearly three-quarters of Labour's support at the last election evaporated, with Mr Ostrowski picking up 6,243 votes, less than half of the 13,591 picked up by Ms Smith.
The size of the anti-Labour swing almost matches that achieved by the Tories last year in the Crewe and Nantwich by-election. Repeated in a general election, it would propel David Cameron into Downing Street with a three-figure majority.
The only consolation for Labour was that it held on to second place as the anti-government vote fractured between the Liberal Democrats, the Greens and the UK Independence Party. Thousands of disillusioned former Labour voters also appeared to have stayed at home.
The collapse in support came amid deep gloom among Labour MPs, who left Westminster yesterday for the parliamentary summer recess.
Their low morale in the wake of the expenses revelations means there is no immediate mood for a fresh attempt to oust Mr Brown, although the question of his leadership could resurface at the Labour conference in September.
The mood was summed up by Andrew Mackinlay, who told his constituency party in Thurrock, Essex, that he would step down at the next election. He said last night: "Despite what's peddled in the media, it's unrelenting work – seven days a week, 52 weeks a year."
Labour MPs from all sections of the party denounced the treatment of Mr Gibson by their leadership. One minister said: "I don't understand why he was deselected by the star chamber – I just don't get it."
Denis MacShane, the former Foreign Office Minister, said: "Ian was very popular in the Commons and in his constituency. It was a misjudgement to expel him and we have paid a very serious price."
Left-winger Alan Simpson protested: "Ian was the victim of a political assassination orchestrated by the party machine in London. The machine wanted a sacrificial victim and who better than to go for than one of Gordon's critics, rather than his friends?"
Tony Wright, chairman of the public administration select committee, said: "He was a victim of a moment when all parties, and all party leaders, were falling over each other to show how tough they were being."
Mr Brown admitted the reverse was "disappointing" but attempted to play down its significance, saying no party "can take a great deal of cheer from this" as votes for main parties fell across the board.
What happened next: The youngest members
At the age of 27, Chloe Smith joins an eclectic list of youngest MPs, from Esmond Harmsworth, who ran Associated Newspapers, and George Grey, killed at Normandy in 1944, to Roy Jenkins and John Profumo. Here, others recall entering the Commons as its youngest member.
*Tony Benn, 1950, aged 25
I was only the Baby of the House for 24 hours – a guy called Tom Teevan took his oath the following day.
Advice: My dad said: "Say what you mean, mean what you say, do what you said you'd do, and don't attack people personally."
*David Steel, 1965, aged 26
There was no induction course. People drew diagrams to show me how to get around.
Advice: Take a little time before making a maiden speech.
*Charles Kennedy, 1983, aged 23
I was in awe of the place. Here were all these famous names, Jim Callaghan, Powell, Michael Foot, Ted Heath, just inches away. Callaghan gave me a tour and told the library staff I needed to be looked after.
Advice: The same as in any walk of life, don't be afraid to ask. People always like to be asked.
*David Lammy, 2000, aged 27
There were lots of press and flash photography. Then you're stood in central lobby and you might as well be a tourist. You have no office, no staff. It's daunting.
Advice: It's a marathon, not a sprint. Take time to reflect on the kind of MP you want to be that's true to your personality. It's important to spend lots of time in the chamber. You have to learn the craft.
*Sarah Teather, 2003, aged 29
It was an amazing spotlight for raising your constituents' concerns – you don't get that again unless you go on to be a frontbencher.
Advice: Your first few months will be a nightmare, but remember you have something that most colleagues won't ever have – a profile.
Interviews by Tom Peck