The BBC, citing an unofficial police source, said on Friday that the tapes had been found during the investigation into the alleged plot.

The metropolitan police did not comment on the report which came after police chiefs said that hundreds of officers from across Britain joined in the investigation, one of the largest in British history.

Staff from nearly every police force in the country are working on the case, and dozens of specialist teams are continuing sweeps of homes, businesses and a stretch of woodland in the investigation into the alleged conspiracy to detonate liquid explosives aboard the planes.

Last week, a US police official said that one "martyrdom" tape had been found by investigators.

The BBC on Thursday quoted an anonymous police source as saying that a suitcase holding "everything you would need to make an improvised device" had been uncovered in High Wycombe.

Police have declined to release details of items recovered during eight days of investigations, but John Reid, the interior minister, told his European counterparts that evidence is mounting against 23 suspects rounded up in a series of pre-dawn raids on August 10.

Police officers have until Monday to question two suspects and until Wednesday to interview 21 others.