Tony Blair's battle to convince the Labour Party he has done the right thing by involving Britain in the Iraq war was made harder yesterday by the threat of US action against Syria.

The prospect of another military strike against an Arab country, however remote the possibility, was enough to spur a former Foreign Office minister to call for an immediate ceasefire.

It is the first time that one of the leading campaigners to stop the war taking place has suggested, since hostilities started, that they should be called off before the coalition has achieved its aim of removing Saddam Hussein.

Doug Henderson, a former minister for Europe and a leading critic of the war, criticised the comments of the US Defence Secretary, Donald Rumsfeld, who had warned the Syrian government that it would be "held responsible" for cross-border trade in military supplies to Iraq.

Mr Henderson said the comments highlighted the danger that war in Iraq would spill into other countries in the region and called for the coalition to declare a ceasefire and withdraw troops from Iraq, using strengthened no-fly zones to contain Saddam in the future.

He told the BBC Radio 4 programme The World at One: "As Arab opinion is lost throughout the Middle East, there will be enormous pressure on young Arab persons to join the Iraqis in different ways, and one of the ways will be supplying arms.

"If the American government then said that was totally unacceptable and they would take military action to prevent it, there really would be a major escalation.

"That's always been my fear: that the attempt to remove Saddam would cause such instability in the Middle East and such ill-feeling between the Muslim world and non-Muslim world. Better ways have to be found to contain him.

"All I can see is a major escalation – with all the risks of involving Syria, Iran and Turkey – or a ceasefire and a withdrawal. And I think a ceasefire and withdrawal is, by far, the better way forward."

Meanwhile, some of the Prime Minister's most hardened opponents were meeting in London yesterday to plan how they could "reclaim" control of the party.

There was also increasing evidence of Labour MPs who have backed Mr Blair coming under threat within their own constituencies – particularly in areas with high concentrations of Muslims.

Mark Seddon, editor of Tribune, called for the creation of a new organisation, Labour Refoundation, to challenge Mr Blair over the political direction of the party.

He was speaking at a meeting in London of about 300 delegates to a Labour Against the War conference. The conference voted by more than two to one for a resolution to replace Mr Blair as leader of the Labour Party.

John McDonnell, a left-wing Labour MP, told them: "This is not about Blair. Forget about Blair. He's not the leader of our party. He does not represent me, he does not represent you, he does not represent Labour parties and trade unions. He's gone."