Margaret Beckett's career goes back to the 1974-9 Labour government
Margaret Beckett has warned Labour rebels to get behind Gordon Brown if they do not want the party to pay a heavy price at the next election.
The former cabinet minister and deputy leader said voters would neither "understand nor forgive" them if they focused on internal rows.
A third MP, Barry Gardiner, has left the government, after joining calls for a leadership election.
Rebel MPs are fighting the party's refusal to send out nomination forms.
Mrs Beckett, who was deputy Labour leader from 1992- 94, and acting Labour leader after John Smith's death in 1994, said Labour MPs and activists must unite behind Mr Brown, who should lead the party into the next general election.
She told the BBC: "The British people will neither understand nor forgive a party that appears to be more concerned with its own internal disputes than with their very real problems."
Mr Brown was also supported by International Development Secretary Douglas Alexander, who told the BBC earlier he believed there was support for Mr Brown "across the party" and it was "unconvincing" to suggest there was significant support for another candidate.
What the prime minister is doing is concentrating on the issues that matter to the country - the situation in the economy, what is happening in the financial markets, Northern Ireland and crime
And former transport minister John Spellar urged Mr Brown to take his cue from the former PM Harold Macmillan and dismiss the rebels as "a little local difficulty".
He said the party could ill afford a possible £2m leadership election and said the "self indulgent antics" of a "pretty small" number of MPs should not divert Mr Brown from the business of government.
Earlier it was announced Mr Gardiner had left his position as special envoy for forestry "by mutual consent", days after Labour vice chair Joan Ryan and junior whip Siobhain McDonagh were sacked for calling for a leadership challenge.
Mr Gardiner had accused Mr Brown at the weekend of "vacillation, loss of international credibility and timorous political manoeuvres that the public cannot understand".
The prime minister's spokesman insisted the PM was focused on the big issues of the economy and public services.
He said: "What the prime minister is doing is concentrating on the issues that matter to the country - the situation in the economy, what is happening in the financial markets, Northern Ireland and crime."
Meanwhile Labour officials have denied breaching the party's constitution, by ignoring calls by 12 MPs for leadership nomination forms to be sent out to all MPs, ahead of next week's party conference.
They need 70 Labour MPs to nominate a challenger if they are to bring about a leadership election. Last year Mr Brown was nominated by all but seven Labour MPs to take over as party leader.
The voters have made their mind up about Gordon and it's not a positive decision
But Labour's general secretary, Ray Collins, has not distributed them, saying the convention of the last 11 years is that they are only sent out to individual MPs upon request.
Critics say this breaches clause 4B 2 of the constitution, which states "where there is no vacancy, nominations shall be sought each year prior to the annual session of the party conference."
Mr Brown is due to attend a meeting of Labour's ruling National Executive Committee on Tuesday, which is expected to discuss whether nomination papers should be sent out.
David Evans, a former assistant general secretary of the party, told BBC Radio 4's World at One programme he was "baffled" as to why nomination papers had not been issued.
"It seems to me the rule is crystal clear. It means that nominations should be sought and it seems to me that in the current circumstances, where a number of Labour MPs have invoked that rule, that there is no room for manoeuvre - that nominations should be issued to the Parliamentary Labour Party."
Earlier Graham Stringer, one of the 12 rebels calling for a leadership election, denied destabilising the party saying: "That is absolutely not true.
"What is happening is not because 12 of us think there should be a leadership challenge, we think there should be a leadership challenge because the government is not doing its job very well."
He added: "The voters have made their mind up about Gordon and it's not a positive decision."
Another rebel, George Howarth told BBC Radio 4's World at One programme that it was "not inconceivable" that Mr Brown might emerge from a leadership contest a stronger prime minister.
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