Heathrow airport has asked airlines to halve the number of passengers they fly into the airport next week to try to minimise disruption caused by a strike.
Its operator BAA warned of "gridlock" and said passengers could face 12-hour delays on Wednesday when immigration officers go on strike over pensions.
BAA said it was holding talks with airlines and the UK Border Agency.
PM David Cameron said it was clear there would be an impact at ports and airports, but borders would be secure.
A 24-hour strike in Portugal against proposed austerity measures has grounded flights and halted public transport.
Hundreds of thousands of workers took part in the action, including air traffic controllers, metro workers, teachers and hospital staff.
A march in the capital, Lisbon, attracted thousands of protesters.
Parliament will vote next week on a deficit reduction plan being imposed as a result of an international bailout.
Portugal is getting 78bn euros (£67bn; $104bn) in emergency funds from the EU and the International Monetary Fund (IMF)
In a separate development on Thursday, the credit ratings agency Fitch downgraded Portugal's debt to junk status, blaming Lisbon's "large fiscal imbalances" as well as the "adverse macroeconomic outlook".
