Up to six US air marshals are now assigned to each US-bound flight from Gatwick airport, according to US reports.

The increase follows fears that terrorists might attempt a co-ordinated series of mid-air explosions, law enforcement officials told the Blotter on ABCNews.com.

One senior law enforcement official told the website: "We're afraid someone in the back is going to mix something or light something up, so air marshals are being placed strategically through the plane."

The stepped-up security began about two weeks ago, based on intelligence reports that another al Qaida hijacking plot was being planned, the officials said.

Security on all flights out of Germany was increased about two months ago, based on similar intelligence, the officials said.

Officials in the US said the federal air marshals altered coverage of domestic flights and cancelled some training classes to have enough marshals for the expanded coverage out of Britain and Germany.

A spokesman for the air marshal programme told the website that coverage in Europe had been increased "but not at the expense of protecting US domestic flights".