RUTH Kelly’s decision to quit the cabinet has ended one dilemma for Gordon Brown — whether to go ahead with an early government reshuffle.

But it has left him with a second problem — how big to make it.

The timing and the reasoning for the Transport Secretary’s announcement has left the Labour Conference, which ended in Manchester yesterday, mystified.

She told him she had informed him of her plans in May, but no one at the party gathering quite understands how the news leaked out late Tuesday night, apparently in a hotel bar.

In her speech to the conference yesterday, she made clear she strongly supported both Mr Brown and his predecessor, Tony Blair.

But rumours remain that she was increasingly unhappy with the direction of the new Prime Minister’s government, and was determined to vote against the Human Fertilisation and Embryology Bill, allowing experiments with women’s eggs, which goes strongly against her Roman Catholic views.

She was allowed to avoid key votes on the legislation earlier this year, but as a cabinet member would have had to have backed the bill in its final stages.

She received a warm reception for saying she remained a fan of Mr Brown, but saying: “However, after committing most of my adult years to politics, I now want to devote more time to my family.

“I owe it to my children and family to put them first. If I do not, then I know that this will be something I will come to deeply regret.”

However, Labour insiders are now worried that her decision to quit the government could lead to more cabinet resignations, with Blairite Business Secretary and fellow North-west MP John Hutton a favourite candidate.

Mr Brown also has a difficult decision of how wide a reshuffle to indulge in.

There has been talk of swapping Chancellor Alistair Darling and Foreign Secretary David Miliband to tie the latter, seen as the Prime Minister’s main leadership rival, into tackling the economic crisis.

There are also suggestions he might want to promote Blairite ministers such as James Purnell and Leigh MP and Culture Secretary Andy Burnham to provide a balance of the different wings of the party in the cabinet.

But others believe that his closest ally — Children Schools and Families Secretary — Ed Balls should be made Chancellor to ensure a common front on the issue.

Ousting Mr Darling, a long-term friend, could both scare the financial markets further and provide a new enemy on the back benches.

Health Secretary Alan Johnson is another possible promotion candidate.

It may be entirely true that 40-year-old Ms Kelly — who has managed to give birth to four children since becoming an MP in 1997 — is sincere in her desire to spend more time with her family. But, then again, she might be wanting to nurture her very narrow majority in Bolton-west.

Whatever her motive, it is certain that for the first time in her political history she has totally dominated a Labour Conference — and left her leader and cabinet colleagues in the shade.