Promotion?: Jack Straw is being tipped for the role of Deputy Prime Minister by loyalists

Gordon Brown is under growing pressure from allies to promote Jack Straw in a September reshuffle.

The Justice Secretary is being tipped for the role of Deputy Prime Minister by loyalists who want Mr Brown to head off any Cabinet coup attempt.

They say he should use the offer of plum jobs to keep potential troublemakers in line.

Mr Straw, one of the longest serving ministers in the Government, is widely seen by MPs as the most credible ringleader for attempts to unseat Mr Brown if things get worse for Labour.

He and Chief Whip Geoff Hoon, along with Chancellor Alistair Darling, are the most likely 'men in grey suits' who might lead a delegation to tell the Prime Minister to stand aside.

Ministers say Mr Brown should use the British place on the European Commission, now held by Peter Mandelson, or even the post of Deputy Prime Minister to reward loyalty.

Jack Straw is being singled out as the obvious deputy, while MPs say Mr Hoon could be lined up to succeed Mr Mandelson.

The Prime Minister is likely to reshuffle his Cabinet in September, before the Labour party conference and the return of Parliament. It will be a key part of what will be portrayed as a drive to pull his leadership out of what many MPs fear is a political nosedive.

Mr Mandelson has said he does not want to be reappointed when a new Commission is chosen next year, leaving Mr Brown with a chance to reward an ally.

MPs are speculating that he could use the post as a way of easing Mr Darling out of the Treasury, although others say it could go to former Health Secretary Patricia Hewitt.

Some Cabinet ministers want to see Mr Darling moved, to revive the Government's economic credibility, but concede it could be a massive own goal.

Mr Hoon, a former MEP and senior minister, appears a natural choice for Brussels, although his departure would force a by- election that Mr Brown is keen to avoid.

Number 10 and Mr Hoon's office deny persistent reports of a rift between the two men over the handling of recent Commons votes on abortion and terrorism.

Downing Street insiders say the Prime Minister has given no thought to switching his team, but many MPs believe he is putting off a traditional summer reshuffle until the autumn as a way of keeping ministers onside.

Labour's dire performance in the opinion polls and uncertainties about the Glasgow East byelection in a fortnight have sparked talk of a difficult summer for Mr Brown. MPs fear he is trapped in a run of political bad luck that is obscuring Government successes on issues such as nuclear energy and terrorism.

Cabinet ministers have told the Daily Mail that Mr Brown has until Labour's conference to persuade his MPs that he should lead them into the next election.

September is emerging as the decisive month that could see Labour frustrations boil over.

The Premier's loyalists fear that the weeks between the end of the summer holidays and the party conference could see an orgy of rumour and plotting. They point out that it was in September 2006 that Mr Brown's own supporters launched the 'coup' that forced Tony Blair to announce he would quit the following year.