Gordon Brown has lost a of trial of strength with Ruth Kelly and is allowing her to miss a crucial vote to relax embryo research next week, The Times has learnt.

Faced with the prospect of having to sack his Transport Secretary for defying the Labour whip next Tuesday, Mr Brown caved in last night.

She was one of three Roman Catholic Cabinet ministers who forced Mr Brown to give Labour MPs a free vote on the Human Fertilisation and Embryology Bill’s second reading in May.

At the time, the concession was presented as a deal under which all ministers would support the Bill at its third reading.

But while Des Browne and Paul Murphy are expected to support the Government next week, Ms Kelly is determined not to vote for the Bill.

Downing Street’s surrender appeared to catch other ministers on the hop. A spokesman for Alan Johnson, the Health Secretary, said: “The expectation is that all ministers will support the Government.”

A spokesman for Ruth Kelly told The Times she had been given permission by Labour whips to spend the day of the vote in Brussels “on ministerial business”.

Mr Brown’s capitulation will do little to restore his battered authority as he faces another by-election. David Marshall, the East Glasgow MP, quit despite frantic efforts to dissuade him. The by-election is likely to be on July 24, after Parliament goes into recess, to try to avoid a humiliating backlash if Labour loses despite a majority of 13,500 in 2005. Mr Marshall, 67, blamed poor health for his decision to stand down after nearly 30 years.

As Mr Brown tried yet again yesterday to get on the front foot, with the launch of the Darzi report on the NHS, backbench MPs were still calling for major policy changes.

The Finance Bill completes its Commons passage today and tomorrow. Labour MPs want Alistair Darling to give a firm pledge that he will rethink plans to apply higher road tax charges on cars bought since 2002.

Others are pressing him to act now to compensate the 1.1 million low-paid people who missed out on his May package to help the losers from the 10p rate abolition. On Thursday the whips face a revolt over MPs’ pay.

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