Seventy Labour MPs would need to back a challenger to Gordon Brown
The Labour Party has moved to see off an attempt by 12 rebel MPs to force a challenge to Gordon Brown's leadership.
Its ruling National Executive Committee has rejected calls to get nomination forms sent out to all Labour MPs before next week's party conference.
And cabinet ministers Harriet Harman, Alan Johnson and Alistair Darling have urged unity in the party.
But Scotland Office minister David Cairns became the fourth MP to leave their government jobs in the row.
Under party rules, the rebels would need to get the support of 70 MPs to prompt a challenge to Gordon Brown's leadership.
Twelve rebels, among them several former ministers, have requested nomination papers be sent out ahead of next week's conference - citing a clause in the party's constitution that nominations "shall be sought each year".
But the NEC agreed with general secretary Ray Collins, after taking legal advice, that they should not be sent out for the leadership when the party is in government, as had been the convention for the past 11 years.
One of the rebels, Janet Anderson, is an NEC member but she was not at Tuesday's meeting.
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The prime minister was there - officials said he was there to deliver a leader's report on government business and to look ahead to the conference - but he did not talk to reporters.
In a statement NEC chair Dianne Hayter said the rules were clear and "internal procedural debates" would not divert the party "from our mission of building a fairer Britain".
She added: "The NEC fully endorses the view of the Labour Party's general secretary and the party's independent legal advisers.
"A leadership election when in government can only be held if requested by a majority of party conference on a card vote. Only Labour MPs can trigger the process and the NEC is confident that most MPs know their responsibilities under the rules."
Leaving a cabinet meeting, Health Secretary Alan Johnson told reporters he was "absolutely confident" that the cabinet was united behind Mr Brown.
He added: "I believe we need unity in the party and I believe the whole history of this party shows the reason why - in the 20th Century, Labour governments were short interludes in Conservative rule, and why we ended up in the situation we did in '97 with so much to do, is because we were disunited in the past."
If they refuse a leadership contest, people will ask 'what has Brown got to be afraid of?
Deputy leader Harriet Harman said the cabinet was united behind Gordon Brown, who was the "best person to lead this country".
Justice Secretary Jack Straw said that while he understood "frustrations" felt by some Labour MPs, he did not think they were "correctly directed".
He told BBC Radio 4's World at One: "I'm absolutely clear Gordon Brown is the man with the experience and the intellect, and the strategy to lead us through these current difficulties in the United Kingdom."
And chairman of the Parliamentary Labour Party Tony Lloyd told the BBC later the number of rebels was actually "very small" and "many, many more MPs" would be needed to mount a serious challenge.
"The challenge actually is damaging, but in its public impact. It's trivial in its real impact and that's the paradox. We've really got to get back to real politics to the sorts of things that do affect people," he said.
Earlier Chancellor Alistair Darling urged Labour colleagues to "get behind" Gordon Brown saying he was the "right person to lead this country and to lead our party".
He told BBC Radio 4's Today programme what concerned people was issues like the collapse of US investment bank Lehman Brothers - which in turn has seen shares fall in leading Asian markets and that was what the government should be concentrating on.
Three MPs - former whip Siobhain McDonagh, former vice chairman Joan Ryan and former special envoy Barry Gardiner - have left their jobs since Friday after calling for a leadership contest.
And on Tuesday afternoon, following growing speculation, Downing Street confirmed it had accepted Mr Cairns's resignation.
Ms McDonagh told the Guardian she did not accept that nomination forms should not be sent out because that had been the convention recently. She added: "If they refuse a leadership contest, people will ask 'what has Brown got to be afraid of?'"
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