Gordon Brown's authority as Prime Minister suffered a major blow last night when a member of his own government broke ranks and called for a leadership challenge.

Gordon Brown's authority as Prime Minister suffered a major blow last night when a member of his own government broke ranks and called for a leadership challenge.

Just 24 hours after Mr Brown sought to seize the political initiative with his energy aid package, Siobhain McDonagh, 48, a whip and a former parliamentary aide to ex-home secretary John Reid, revealed that she had requested leadership nomination papers from Labour HQ because she wanted a debate about the future of her party. "We need to clear the air," she declared.

It is thought at least four other Labour MPs and ex-ministers - Frank Field, Joan Ryan, George Howarth and Graham Stringer - have also called for the official papers.

While Labour HQ sought to brush the matter aside as of no importance, it was interpreted by one UK Government minister as very significant, telling The Herald: "This is the beginning of the end for Gordon."

No-one expects the PM to face a serious challenge to his position at the Labour Party conference in Manchester in two weeks' time as the names of 70 MPs - 20% of the total - are needed to spark a ballot. However, the minister noted: "This is not meant as a serious challenge but it's a group of MPs who are sending a signal that Gordon's leadership is no longer tenable."

He added: "This is very serious for Gordon. I'm convinced this will be his last party conference as leader."

In September 2006, a group of parliamentary aides resigned their posts in protest at Tony Blair's refusal to stand down as Prime Minister. While the attempt did not work immediately, Mr Blair did go earlier than he wanted to, the following summer.

Within an hour of Ms McDonagh making her call, Downing St announced that she had been replaced in the whips' office by Dawn Butler, the MP for Brent South. No 10 denied she had been sacked, pointing out how in a conversation with Geoff Hoon, the Chief Whip, she acknowledged "her actions were a resigning matter".

The matter of a leadership contest now looks certain to be discussed on Tuesday at a meeting of the National Executive Committee, Labour's governing body.

The move by Ms McDonagh, the MP for Mitcham and Morden in London and one of a few MPs who did not nominate Mr Brown last year for the leadership, is poignant because she has never voted against the UK Government in her 11 years at Westminster.

"I've thought about this long and hard," she explained. "Every morning, I get up and think, well, if it's not going to be anybody else, then it has to be people like me to say, come on, we need to have a debate. I thought whatever the consequences for me personally, I've got to argue my corner."

Ms McDonagh, making clear she had no-one in mind to stand against the Prime Minister, added: "We need a leadership election. Anybody who wants to stand should stand. We should have a discussion about what direction the party should be going in."

Her call was followed last night by an article penned by 12 Labour back-bench MPs, including six former ministers, urging Mr Brown to develop "a convincing new narrative" to restore Labour's fortunes.

Writing in the Labour conference edition of Progress magazine, they say that Labour has "no explanation yet" as to how it would "steer the economy through the troubled waters ahead". They add that there is a "yawning chasm", which Labour needs to fill, or the UK Government will suffer a "hammer blow".

Last night, opposition parties made hay with Ms McDonagh's open challenge to the Prime Minister.

Angus Robertson, the SNP leader at Westminster, said: "As one Labour leadership battle ends in Scotland, it looks like another one is starting in London."

Chris Grayling for the Tories claimed Labour was "degenerating into a state of civil war", while Danny Alexander for the Liberal Democrats added: "Labour MPs are so busy fighting amongst themselves that they can no longer govern properly."