Some British Airways passengers expecting to fly this weekend to Florida and Kenya on state-of-the-art jets will instead find themselves squeezed into a Portuguese charter aircraft.

The airline says that the Boeing 777 normally used for the Gatwick-Tampa service will be replaced by a 767 charter jet operated by EuroAtlantic Airways of Sintra, near Lisbon.

A BA spokesman said the change is "a knock-on effect" from the severe winter weather that affected the north--east US last month, causing dozens of flight cancellations and disrupting cabin-crew rosters. "Instead of cancelling the flight we chartered in another company," said the spokesman.

The flights are "wet-leased": pilots and cabin crew are supplied by EuroAtlantic. Business-class passengers are to be handed pounds 250 in compensation when they turn up at Gatwick. Travellers in economy-class passengers get nothing, because BA describes the service as "broadly similar" to its own economy product.

Yet the seat pitch in EuroAtlantic's main cabin is an inch less than the BA standard. In addition, the Portuguese jet cannot fly as fast as the 777, extending the journey to around 10 hours.

Contributors to the online air-travel forum, FlyerTalk, have condemned the change of plane. One wrote "It's one thing to replace an aircraft, quite another to replace it with a seriously substandard (from a passenger perspective) machine, without any achievable advance warning or meaningful compensation." BA's spokesman said " We'd like to apologise to all customers affected by the change of aircraft. We're doing all we can to resolve it as quickly as possible".

The airline is trying to contact passengers to advise them of the aircraft, change, and offer the chance to re-book, re-route to another BA destination or claim a refund. Normal service to Florida is due to resume on Monday.

Travellers on some flights from London to the Kenyan capital will also travel aboard a charter aircraft. The airline said that "a maintenance overrun on the 767 fleet" had caused it to replace some scheduled Heathrow- Nairobi services with a Lockheed Tristar, also chartered from EuroAtlantic.