POWERFUL cabinet ministers who see themselves as potential successors to Gordon Brown are secretly plotting against him despite public protestations of loyalty.

Even as Brown met yesterday at No 10 with Barack Obama, the visiting US Democratic presidential candidate, the prime minister was an increasingly isolated figure whose days in office are now being measured in weeks.

Senior MPs are running a campaign on behalf of Jack Straw, the justice secretary, to collect names for a possible leadership bid.

A backbencher has told how he had been approached by George Howarth, the former Home Office minister and a friend of Straw.

“George told me he was collecting names for Jack,” said the MP. “He said Jack was ready to tell Gordon the game was up, if there was enough support.”

Other MPs have revealed how a “Lancashire mafia” of members allied to Straw, who represents Blackburn, have met colleagues in the Commons tea room to ask if they would support him as a “save the party” leader. They have also been calling colleagues to prepare the ground for a possible autumn coup.

Angry Labour MPs last night called on Brown to sack any minister found plotting against the leadership.

“It’s positioning,” said one. “People have been ringing round. But they are part of the problem, not part of the answer. Jack Straw tried to back-stab Blair and now he is back-stabbing Brown.”

The private manoeuvring contrasts with assurances by Straw’s aides that he remains loyal to Brown after the Glasgow East by-election debacle. Straw left yesterday for a holiday in America and was unavailable for comment.

Howarth confirmed last night that he had concerns about Brown’s leadership: “Everybody’s got to think long and hard about a number of issues, including policy, the party’s popularity and the leadership.”

However, he insisted that he was not acting on Straw’s behalf. “Jack is not up to anything,” he said. “If anyone thinks I am collecting names for him, they are mistaken.”

Straw’s spokesman said: “These people are not acting on Jack’s behalf. Jack has not sanctioned this behaviour. Jack’s view is that his primary loyalty is to the Labour party and he believes this sort of thing is not in Labour’s interests.”

Hazel Blears, the communities and local government secretary, appealed for unity: “When your party is in a tough way you have a choice. Tear each other apart or pull together - and it is vital we do exactly that.”

Straw is just one of several cabinet ministers whose loyalty to Brown is in doubt. MPs close to Harriet Harman, Labour’s deputy leader, have also been sounding out colleagues about her chances of winning a contest should Brown be ousted in the autumn. “Harriet’s behaviour is of somebody who really believes she would be the women’s candidate,” said an MP.

David Miliband, the foreign secretary and bookies’ favourite to succeed Brown, was spotted on Friday evening drinking at a bar with union leaders, who will play a key role in deciding the succession.

Miliband, a minister not normally known for his love of late-night drinking, was chatting to Tony Woodley, a joint general secretary of Unite, Britain’s biggest union, and John Hannett, general secretary of Usdaw, the shopworkers’ union.

The meetings, which took place during the Labour Policy Forum at Warwick University, came amid frenzied discussion among ministers, union bosses and party activists about the fate of Brown.

“People were wondering what Miliband was doing there,” said a party official. “He does not have a major role in the policy forum process and nor is he close to the trade union movement. But then it cannot do him any harm to win friends among the party’s paymasters.” Miliband’s allies insist that his conversations were about policy rather than the leadership.

The cabinet manoeuvring comes amid mounting evidence that Brown will face a formal challenge as early as the first week of September. Blairite former ministers hostile to Brown have drawn up a “grid” for a possible coup, with the plotters issuing an ultimatum to cabinet ministers that they would have a week to force Brown to quit. If he stayed, phase two would begin on September 8 with the resignations of junior ministers and parliamentary aides. Backbenchers would publicly call for Brown to resign.

Cabinet sources are also considering a “death with dignity” scenario where Brown would be offered a senior international post, possibly involving African development.

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