Portugal has become the third nation to ask the European Union for financial assistance.
Caretaker Prime Minister Jose Socrates said the country was "at too much risk that it shouldn't be exposed to".
Portugal, which has seen ratings agencies downgrade its debt in recent weeks, follows Greece and the Irish Republic in seeking a bail-out.
Socrates did not reveal how much aid the country would ask for, but the BBC estimates it could be as much as £70bn.
"I always said asking for foreign aid would be the final way to go but we have reached the moment," Socrates said. "Above all, it's in the national interest."
Reports suggest that, despite an explicit guarantee by the Chancellor, George Osborne, of no further aid to the eurozone after a loan to Ireland last year, the UK could be liable for as much as £4.4bn to help fund the rescue.
The Independent put the UK's liability a little lower, at £3bn, saying part of that comes from an "obscure and dwindling" EU Commission fund the UK remains signed up to.
An additional bill derives from the UK's membership of the IMF, as all these rescues are partly funded through that body.
The Portuguese government had until now resisted asking for aid but last week admitted it had missed its 2010 budget deficit target.
European Commission President Jose Manuel Barroso said in a statement Portugal's request would be processed "in the swiftest possible manner, according to the rules applicable".
He also reaffirmed his "confidence in Portugal's capacity to overcome the present difficulties, with the solidarity of its partners".
'Life catches up with us in the end'
‘Personalised Predictive Analysis’ tool
Summer series continues
Both start in August