Caretaker Prime Minister, Jose Socrates, said the country was "at too much risk that it shouldn't be exposed to" and admitted the country needed an injection of cash after saying last week it had missed its 2010 budget deficit target.
"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."
He did not reveal the extent of the request but the BBC estimates it could be up to £70bn – meaning the UK could be liable for as much as £4.4bn, despite a guarantee from Chancellor George Osborne that the Irish bail out would be Britain’s last to a failing Eurozone country.
The UK’s contribution will derive in part from its membership of the International Monetary Fund.
Portugal was downgraded by ratings agencies in recent weeks and it follows both Greece and Ireland in seeking a Eurozone bail out.
European Commission President, Jose Manuel Barroso, said Portugal's request would be processed "in the swiftest possible manner, according to the rules applicable".
He added he had confidence in “Portugal's capacity to overcome the present difficulties with the solidarity of its partners".
