Gordon Brown suffered a serious blow to his authority last night when one of his own whips called for a contest to replace him as Labour leader.

Mr Brown was forced to sack Siobhain McDonagh from the Government after she confirmed that she wanted to trigger a leadership election at the party’s conference in a little over a week.

Speaking to The Times, Ms McDonagh said she had never before done anything disloyal but she believed that ordinary Labour members should be given a say on the party’s future: “I just thought that it was important that this was out in the open.”

Asked whether she thought that Mr Brown could win the next election she said: “I don’t know.”

Ms McDonagh, the MP for Mitcham & Morden, whose sister, Margaret McDonagh, was Labour general secretary under Tony Blair, is one of eight or so MPs seeking to exploit party rules to trigger a leadership election.

The MPs, most of whom are former close allies of Mr Blair, have written to Labour’s general secretary demanding nomination papers for a leadership election.

Others MPs involved include Barry Gardiner, Jim Dowd and Joan Ryan, a Labour vice-chairman. Another rebel, Graham Stringer, said that he expected more people to send letters asking for a vote at conference: “There is a serious level of anxiety that the leadership of the party isn’t getting this right.”

Ms McDonagh was contacted by Geoff Hoon, the Chief Whip, shortly after midday. At around 4pm she said it was unclear whether she would be asked to resign or be sacked but that she was unafraid of losing her ministerial salary of £65,642. “This matters more to me than having a junior job in Government.” An hour later she was sacked.

Later the Prime Minister received a second, coordinated, attack from inside his party when twelve back-benchers, including six former ministers, gave warning that Mr Brown’s Government was suffering a “malaise” and Labour is heading for a “hammer blow” at the next election.

MPs including Patricia Hewitt, the former Health Secretary, urged him to find simple messages to encapsulate policies, like Harold Wilson’s “pound in your pocket” and Margaret Thatcher’s comparison of the economy to a household budget.

Writing in Progress magazine, they said that Labour had “no explanation yet” about how it would “steer the economy through the troubled waters ahead”, adding that “one-off taxes and pay-outs, no matter how justified in their own terms, do not amount to a strategy”.

The other former ministers on the list were Janet Anderson and Stephen Ladyman, former transport ministers; Karen Buck and George Howarth, former Home Office ministers; and Paddy Tipping, former deputy leader of the House. The backbench MPs were Eric Joyce, Sally Keeble, Martin Linton, Shona McIsaac, Margaret Moran and Tom Levitt.

The Labour National Executive Committee meets on Tuesday to discuss preparations for the party conference. The Prime Minister’s allies are resisting efforts to place the MPs’ demands for nomination papers on the meeting’s agenda.

Mr Brown has been braced for a fresh attack before the Manchester gathering. Charles Clarke, the former Home Secretary, effectively gave the Prime Minister until the Glenrothes by-election, expected in early November, to improve.

Mr Clarke said that Labour faced certain defeat at the next election unless the Prime Minister improved his performance, and called on him to consider stepping down “with honour”.

Jack Straw, the Justice Secretary, admitted that Mr Brown’s administration required a “rethink” but cautioned against an attempted coup.

Ms McDonagh’s replacement will be Dawn Butler, MP for Brent South, who will become the first black female minister in the Commons.

Exit strategies

How to oust a Labour leader

When Labour is in government, a leadership election is triggered if a majority of the delegates at party conference vote for one. Such a vote is held if 20 per cent of MPs – currently 71 – ask for one. Nomination papers have not been sent to MPs for 11 years, but now some are asking for them. A party spokesman said: “This procedure has been followed for the last 11 years and we are confident that most members of the Parliamentary Labour Party are fully aware of their responsibilities under the rules to trigger the process, which has not required the issuing of nominations forms.”

How to oust a Tory leader

A Conservative leader faces an election if 15 per cent of the parliamentary party lodge a letter of no confidence with Sir Michael Spicer, chairman of the backbench 1922 Committee. The identity of people who send in letters remains secret and the chair alone knows both the identity and number. This technique was used to oust Iain Duncan Smith in October 2003 when 25 MPs requested a vote of confidence. Mr Duncan Smith lost the vote the next day. Michael Howard, his successor, tried to change the rules but this was rejected by party members.

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