The size of any bailout for Portugal is estimated at between 75bn to 85bn euros (£65bn to £75bn).
European finance ministers will also discuss possible conditions that could be attached to it such as austerity measures.
The make-up of any package, as well as interest rates, will be debated.
Portuguese Prime Minister Jose Socrates resigned last month and warned of grave consequences for the country after parliament rejected his government’s latest austerity measures aimed at avoiding a bailout.
Sources said the final deal for Portugal would be similar to that offered to Greece and Ireland. But the British Government will not be offering Portugal a bilateral loan as it did to Ireland.
Portugal’s plea for help came as the European Central Bank put up interest rates by a quarter of one per cent, putting further pressure on ailing economies within the eurozone.
But Labour accused him of “desperate scaremongering”, warning that the coalition Government’s austerity programme had in fact put the UK into the group of “slow-growth” economies alongside Portugal.
And there was anger among Conservative MPs at the prospect of the UK being forced to help rescue another member of the eurozone, having already contributed emergency funding to Ireland.
The terms of a deal signed by Mr Osborne’s predecessor Alistair Darling in the dying days of the Labour government, means that Britain is committed to contributing a share of any bailout provided before 2013 under the European Financial Stability Mechanism (EFSM).
Officials at the Treasury have confirmed Britain could be required to underwrite a loan of up to about £4.4bn – 13.6 per cent of the 37.5bn euros remaining in the EFSM fund – as well as 4.5 per cent of any IMF loan to Portugal.
Eurosceptics argued any Portuguese bailout should be funded from the separate 440bn euro European Financial Stability Facility, which involves only eurozone countries and imposes no liability on the UK.