EU commission chief Jose Manuel Barroso, in a clear warning to Britain, reiterated a call Thursday for EU leaders to avoid any "unilateral" decisions on the bloc's crisis ahead of a mid-June summit.

Barroso, speaking after talks with European Parliament party chiefs, said all EU leaders should show "caution" ahead of the June 16-17 summit, expected to decide how to respond to French and Dutch rejections of the EU constitution.

"What I am asking for now is that political leaders, in particular government chiefs, not take individual, or unilaterial decisions," he told reporters.

"I ask political leaders to show responsibility, to show caution," he added.

He was speaking after senior officials in London were quoted as saying that Foreign Secretary Jack Straw would announce on Monday that Britain's referendum on the EU constitution will be put on hold.

Straw will tell parliament that the European Union Bill paving the way for the referendum is to be put on ice indefinitely following the French and Dutch "no" votes, they told Britain's domestic Press Association news agency.

Barroso inadvertently confirmed that his warning against unilateral action was aimed notably at Britain, when he was asked about an unconfirmed report that London had pledged to take no such action ahead of the EU summit.

He said he was unaware of this report, but called it "excellent" news if confirmed. "That means the message has been understood, so I welcome it," he said.

The EU chief added that he still believed the 25-nation EU could overcome the crisis. "It is not the first time that Europe has had a problem, it will not be the last time," he said.