A major speech Mr Brown is preparing to deliver at Sunday's Welsh Labour conference will further fuel expectations that he will run.
In a brave effort to contain leadership manoeuvrings during the European election campaign, last night's Shadow Cabinet resolved that candidacies should not be made public until after 9 June. But Mr Brown's supporters were in little doubt yesterday that he is bound to use the occasion to spotlight - however indirectly - his leadership qualities.
In what will be the first major party event following John Smith's funeral on Friday, Mr Brown will attract the undivided attention of the media.
The developments will aggravate the fears of some Labour moderates that a contest involving both Mr Brown and Tony Blair, the early front- runner, risking a defeat for the 'modernising' wing, is on the cards.
Senior Labour figures and Scottish MPs are among those who would back Mr Brown if he stood. There was even a suggestion that Mr Brown would pick up second-choice votes from left-wing supporters of John Prescott, the employment spokesman, and win. But he is also being urged to go ahead by left- wingers who believe his candidacy would split the modernising right and let in a leader from the left.
Mr Brown's camp is well aware of the danger of false friends forcing his hand. But the potential for splits among the modernising MPs was briefly exposed yesterday. One Brown supporter, Nigel Griffiths, the MP for Edinburgh South, angrily accused another, Peter Mandelson, the MP for Hartlepool, of being a covert Blair canvasser.
Mr Griffiths hurriedly withdrew the accusation, but the air is thick with mutual suspicion.
On the left, it had seemed that Robin Cook, the trade and industry spokesman, was poised to decide against standing. But in the past 48 hours there have been strong messages of support from MPs and some Shadow Cabinet members.
Further encouragement came yesterday from a straw poll of more than 70 MPs by Tribune, the left-wing weekly, suggesting that Mr Blair, the shadow Home Secretary, and Mr Cook are the strongest contenders.
The poll is a less than perfect guide but it claimed there was concern about Mr Prescott's suitability as a potential prime minister.
There is an incipient campaign for Mr Prescott to have the chance to couple a leadership stand with a play for the deputy leadership.
Detailed discussions on timing, and the voting system, are due to be finalised next Wednesday.
But, while nominations for the deputy leadership, which can be contested at the autumn party conference, do not close until 8 July, those for the leadership are likely to close in mid-June, closing off the option of an earlier challenge to Mrs Beckett unless she decides, as some women MPs and hard-left Campaign Group MPs would like, to stand as leader herself.
But Mrs Beckett, above all, is highly unlikely to deviate from the Shadow Cabinet's ban on public pronouncements until after 9 June, thereby keeping opponents dangling.
Isabel Hilton, page 19