José Socrates, Portugal's prime minister, has appealed to opposition parties to reassure financial markets over the country's sovereign debt difficulties by expressing support for a tough austerity budget.

Speaking shortly before details of the 2011 budget proposals were due to be presented to parliament on Friday night, he warned that rejecting the bill would plunge the country into a damaging political crisis.

Mr Sócrates has said his minority Socialist government will resign if it is defeated in the budget vote. But the centre-right Social Democrats (PSD), the main opposition party, have refused to give any assurances that they will allow the bill to pass.

"This uncertainly is extremely harmful to the country," Mr Sócrates told parliament. "It is time for the PSD to end the taboo on how they will vote."

But PSD leaders said they would not make any commitment until they had studied the proposals in detail, accusing the government of trying to press them into approving inadequate measures.