Gordon Brown's leadership has been under the spotlight for several months
A senior minister is ready to quit over concerns about Gordon Brown's leadership, the BBC has learned.
The unnamed minister said he believed the prime minister had failed to show the vision needed to lead Labour.
The minister of state said he could not continue to say Mr Brown was the man to lead the party to victory when he did not believe that was the case.
Meanwhile, former Foreign Secretary Margaret Beckett has warned Labour rebels to get behind Mr Brown.
The minister said he had not come to a firm decision on whether to resign yet but he believed it was impossible for the government to continue without the question over its leadership being resolved.
He told the BBC: "There just comes a point where you say, 'I can't go on lying', you can't go on saying 'I think Gordon Brown is the man to lead us to victory' when you don't believe it."
BBC News political correspondent David Thompson said questions surrounding Mr Brown's leadership were growing.
The minister said he was thinking very hard about resigning and was close to doing so, our correspondent added.
BBC political editor Nick Robinson said there was talk among those who wanted to see the prime minister go that as many as five or six ministers were ready to resign.
So far more than a dozen MPs have called for a leadership contest, including former Labour minister Fiona Mactaggart and party vice-chairman Joan Ryan.
On Monday, Gordon Brown's Forestry Envoy Barry Gardiner became the third MP to leave his government post, days after Ms Ryan and junior whip Siobhain McDonagh were sacked for calling for a leadership challenge.
'Loss of credibility'
Mr Gardiner had published an article complaining of "vacillation, loss of international credibility and timorous political manoeuvres".
Downing Street said his departure had occurred "by mutual consent"
The Labour rebels are calling for leadership nomination papers to be sent out to all of the party's MPs to see if there is enough support for an election.
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.
Ms Beckett and other senior Labour figures have been rallying to Mr Brown's defence.
She said voters would neither "understand nor forgive" the party if they focused on internal rows.
Support 'across the party'
International Development Secretary Douglas Alexander told the BBC he believed there was support for Mr Brown "across the party" and it was "unconvincing" to suggest there was significant support for another candidate.
Former transport minister John Spellar urged Mr Brown to take his cue from former prime minister Harold Macmillan and dismiss the rebels as "a little local difficulty".
The prime minister's spokesman insisted Mr Brown 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."