Surprise departure of close ally adds to leader's vulnerability

One of Gordon Brown's closest allies is to stand down as a minister as the rush by Labour MPs towards the exit turns into a stampede.

Tom Watson is to add to the pressure on the Prime Minister by relinquishing his post as Cabinet Office Minister in the reshuffle expected within days.

Two senior MPs – Patricia Hewitt, the former Health Secretary, and Beverley Hughes, the Children's Minister – announced they would leave Parliament altogether at the next election.

Mr Watson's decision came as a surprise as he has always been a close confidant of the Prime Minister. He was part of a close-knit team of Brown loyalists that included Damian McBride, who was sacked two months ago over emails he sent discussing plans to smear senior Tories. But Mr Watson, who was not copied into the emails, may have concluded he is more useful on the back benches shoring up support for Mr Brown among disillusioned Labour MPs. Ms Hughes, who has spent nine years in the Government in two stints as a minister, will join Mr Watson and Jacqui Smith, the Home Secretary, in bowing out at the reshuffle. Unlike them, however, she is also resigning as the MP for Stretford and Urmston, Greater Manchester, at the election. She cited "family reasons" for her decision to quit politics and denied it was linked to the expenses furore.

Ms Hewitt served in Tony Blair's cabinet for six years and will step down as MP for Leicester West at the election. She said she wanted to spend more time with her family and concentrate on charity work in India.

Last night the Labour party was rocked by further revelations on expenses, after The Daily Telegraph revealed that Bob Ainswor th, the Armed Forces Minister, claimed nearly £6,000 for the redecoration of his designated second home as well as a £1,000 LCD television and £2,225 sofa.

Mr Ainsworth defended his actions, saying he had, "acted within the spirit and letter" of the law, though the rules were "bad" and needed to be changed.

David Chaytor, one of the Labour MPs facing investigation over his expenses claims, announced he would not contest his Bury North constituency at the election. Yesterday he was barred from standing for Labour after the party held a "star chamber" inquiry into claims from the taxpayer for a mortgage that had already been paid off. Mr Chaytor said his "priority must be to explain my errors following allegations over the use of parliamentary allowances". He is to repay nearly £13,000 he claimed by continuing to submit £1,175 monthly bills for mortgage interest months after the loan was paid off.

Last night, Labour announced that Mr Gibson, who claimed £80,000 for a flat where his daughter lived with her partner, would be barred from defending his Norwich North seat. He had previously said he was ready to stand down if voters in his constituency wanted him to quit.

Another Labour backbencher, Jim Devine, is facing de-selection over allegations that he submitted receipts from a firm that may not have existed. He insisted he had been subjected to malicious leaks of his expenses claims and was going before the party's National Executive Committee to explain his actions.

The stream of resignations at the next election reflects a growing anxiety among Labour MPs that the party faces a hammering at the polls. Other MPs – in common with some Tories – privately protest that the reputation of politicians has been so tarnished they do not want to remain in the Commons.

The result is that the Labour benches face their biggest upheaval for decades as a series of normally safe Labour seats unexpectedly require new candidates.