It was described as the worst week for the Labour Party since the general election. The furore caused by Harriet Harman’s decision to send her son to a selective school soon became a leadership issue when Tony Blair came to her defence despite the fury of other party members, including education spokesman David Blunkett.

It gave Labour’s opponents ample ammunition to shoot holes in its education policy and tar the party with accusations of hypocrisy. And for the Tories, the timing was perfect.

On Monday it was the second reading of an Education Bill, already bowdlerised by bishops’ crooks, forcing a controversial nursery voucher scheme on a less than enthusiastic Education and Employment Secretary and ending in what most observers view as a half-hearted measure to revive the Government’s flagging opting-out policy.

But on the night it was Gillian Shephard who was able to face her Labour opponents and watch them wriggle in their seats when she said their education policy was in disarray.

Several minutes into the session Tony Marlow, Conservative MP for Northampton North, stood up and beaming an expression of smug glee shared by most others on his side of the chamber suggested three cheers for the member for Peckham.

It was not the last time the south London constituency’s representative - Harriet Harman - was mentioned. Ms Harman’s decision to send her son to St Olave’s grammar school 10 miles away in Orpington, Bromley, was the best 50th birthday present Mrs Shephard could have hoped for.

During the day the Labour party tried to maintain radio and TV silence on the subject. David Blunkett’s office had planned the early morning offensive on Radio 4‘s Today with figures showing Pounds 290 was being spent per child on the bureaucracy required in the pilot nursery voucher schemes - a total of Pounds 187 million. But it was not to be.

The Labour education spokes-man had been put in a very difficult position. At the last Labour party conference he said: “Read my lips. No selection, either by examination or interview under a Labour government.”

When his leader Tony Blair sent his son to the London Oratory grant-maintained school, Mr Blunkett was left to make the excuses. Many party supporters who had been through the opting-out campaigns with their often bitter disputes felt betrayed.

But when Harriet Harman and her husband Jack Dromey, a senior Transport and General Workers’ Union official, who have one son at the London Oratory, elected to send 11-year-old Joe to a selective grammar, it must have stuck in Mr Blunkett’s throat to attempt to bail her out.

With the attack on the Bill reduced to a farce, he was forced to say, after Tory backbencher after Tory backbencher taunted him: “Every parent in every community, whether they are a Member of Parliament or not, should have the right to exercise a preference for their child to go to the school of their choice.

“That preference should not be blocked by any mechanism that prevents a child entering that school - either on its prior attainment at 11 or on the interview of its parents by a head . . . That’s why we will remain against selection. ”

David Blunkett had been told about Ms Harman’s decision some weeks ago, but when the Labour party got wind of the Mail on Sunday running the story the day before the debate, Ms Harman told a number of the Saturday papers. Days later the story was still on the front pages.

In the teeth of a storm of internal party opposition to her decision Mr Blair, himself seen as vulnerable to charges of hypocrisy after choosing the Oratory for his eldest son, has stuck by her again, expecting others in the party to minimise the damage, despite her apparent breach of party policy. She may now have to fight hard to retain her Shadow Cabinet place in the autumn.

Mr Blunkett managed to raise a few laughs when he attempted to go back to the original script by describing Mrs Shephard as a Norfolk market gardener “walking behind the Prime Minister like someone following a rag-and-bone man with a diarrhoeic horse shovelling up the mess and actually following every idea the Prime Minister comes out with on education”.

Roy Hattersley had already attacked the Government for dropping the Prime Minister’s proposal for a fast-track route to grant-maintained status after the church schools’ lobby made their total opposition known. But it was an uphill task. The Tory backbenchers bounced around on the green leather upholstery like Sesame Street characters on Ecstasy, one screaming he was a secondary modern boy, another flashing his St Olave’s school tie.

It was left to George Walden (Con Buckingham) to make the first serious contribution, asking the amount of “dead-weight money” (the number of Pounds 1,100 vouchers given to parents who already pay for their four-year-olds’ education.

The Government won the vote with a majority of 32 votes, with Conservative MPs Michael Stern (Bristol NW) voting against and Iain Mills (Meriden) abstaining.