Gordon Brown's authority was hanging in the balance today after the defence secretary, John Hutton, became the fourth cabinet minister to tender his resignation.

As the prime minister began a make or break reshuffle in an attempt to shore up his increasingly beleaguered premiership, Hutton announced he was to stand down from the government for "family reasons". He will leave parliament at the next general election.

His decision is another blow to Brown, not least because Hutton is the fourth Blairite cabinet minister to stand down in four days.

His resignation follows those of Jacqui Smith, Hazel Blears and James Purnell.

Alan Johnson, tipped as the most likely successor to Brown should he resign as prime minister, is to be rewarded for his loyalty in recent days with the key post of home secretary, moving from health.

But in a blow to Ed Balls, the children's secretary and Brown's key ally, the weakened prime minister was expected to allow Alistair Darling to retain his post as chancellor.

The justice secretary, Jack Straw, was also expected to remain in place, while Yvette Cooper was tipped to be promoted from chief secretary to the Treasury.

Today, more backbenchers echoed Purnell's call for Brown to go amid suggestions that up to 100 Labour MPs were prepared to call for him to quit.

The first results from yesterday's local elections compounded the crisis surrounding Downing Street, with the party losing 23 seats and holding only 20.

Bristol fell into Liberal Democrat hands after Labour lost eight of the ten seats it was contesting in the city.

But after the resignation of Purnell, the work and pensions secretary, as the polls closed at 10pm yesterday, Brown was given a boost by other cabinet ministers. .

Johnson released a short statement to the Press Association, insisting: "I continue to believe that Gordon Brown is the best man for the job.

"It is vital now, more than ever, that we unite for the sake of the party and the government."

David Miliband, a close ally of Purnell's, indicated that he would not be resigning from the government. He is expected to remain as the foreign secretary.

Meanwhile, Darling – who endured speculation earlier this week that he was going to be removed from the Treasury – has yet to comment.

Last night, Purnell gave the prime minister only moments notice of his departure as work and pensions secretary, which was announced in an open letter published in the media.

The leading Blairite plunged the government deeper into crisis by saying Brown's removal was needed to give Labour "a fighting chance" of victory at the general election.

"I now believe that your continued leadership makes a Conservative victory more, not less, likely," he wrote.

"We need to show that we are prepared to fight to be a credible government and have the courage to offer an alternative future.

"I am therefore calling on you to stand aside to give our party a fighting chance of winning. As such, I am resigning from government."

His move was welcomed by backbenchers including Siobhain McDonagh, who was sacked as a whip after calling for a leadership election last year, and the Nottingham North MP, Graham Allen.

Allen said he hoped Brown would "take the honourable way out so that the party can progress with a leadership election".

The Labour backbencher Paul Farrelly said Purnell's actions were "courageous".

"I'm not particularly close to him personally or politically, but I think what he's done is a courageous act, and reluctantly, and very sadly, his assessment is correct," he told BBC Radio 4's Today programme.

The senior backbencher Barry Sheerman, the chairman of the schools select committee, called for a secret ballot of Labour MPs on Brown's future, which he predicted would result in a majority vote for the prime minister to go.

The Conservatives demanded an immediate general election, calling on Brown to go to Buckingham Palace today to ask for a dissolution of parliament.

David Cameron, the Tory leader, said the government was "falling apart in front of our eyes".

In a statement, Downing Street said Brown was "disappointed" by Purnell's decision but was giving his "undivided attention" to the big challenges facing the country.

Those defending the prime minister and criticising Purnell included the cabinet ministers Straw, John Hutton, Geoff Hoon, Shaun Woodward and Andy Burnham, as well as the Cabinet Office minister, Liam Byrne, the housing minister, Margaret Beckett, and a host of whips, junior ministers and backbenchers.

Cooper said Purnell had been guilty of a "misjudgment".

She said: "I just profoundly disagree with James. I've known James a long time, for 20 years, and I just think he's made a misjudgment about this.

"I also think he doesn't speak for the cabinet, for the Labour party, for the ordinary members that I was out door-knocking with yesterday evening.

"I'm not going to pretend this isn't a difficult morning for us, but I think that Gordon Brown, all the work he's been doing on the economy, all the work to deal with the expenses problems, he's the right person to lead us through this."

Douglas Alexander, the international development secretary, told BBC Radio 4: "I believe Gordon Brown is the right person to lead the party.

"At the moment, the country would not look favourable on a party that turned in on itself ... my interest is what ultimately is the right thing for the country.

"I do not believe the best interests of the party are served if Gordon Brown is removed as leader."

But many of the supportive emails issued were almost identically worded, fuelling suspicions that they were part of a Downing Street effort to bolster Brown's position.

However, other MPs were clearly outraged at the cabinet infighting at a time when grassroots activists were out campaigning for Labour in the elections.

The Morecambe and Lunesdale MP, Geraldine Smith, said: "I am absolutely furious at the gross disloyalty of these people.

"Most Labour MPs in the parliamentary Labour party are still backing Gordon Brown, and there is a small bunch of malcontents who are out to do as much damage as possible. It is disgraceful."

And the former deputy prime minister John Prescott used a message on the Labourhome blog to dismiss Purnell as "not so much a Blairite as a careerite".