LONDON (AP) _ Thousands of valuable racing pigeons have failed to find their way home from France since they flew off June 25 in a cross-Channel competition.

Pigeon owners believe the pigeons' internal navigation systems were thrown off course, possibly by microwave ovens, radar signals, radio frequencies or a solar flare.

Mary Hamilton, secretary of the Sheffield and District Homing Pigeon Federation, called it ''a disaster on a vast scale.''

More than 5,000 birds were released in Nevers, France, for what should have been a 14-hour sprint home to northern England, where pigeon racing is a popular betting sport.

But by Friday night, only 1,500 had arrived home.

''The really appalling thing, from the pigeon fancier's point of view, is that these are the best birds,'' said ornithologist Chris Mead. Some of the pigeons are worth more than $54,300.

The pigeons use the Earth's magnetic field to navigate, and several theories have emerged to explain how they got lost.

Derek Towers, secretary and manager of the Up North Combine, thought a solar flare - an explosion of gases on the surface of the sun - upset the birds.

''Electrically charged particles are sent out through space and when they reach us it disorients the pigeons,'' he said. ''This happened once before, years ago, when a magnetic pulse went through the atmosphere.''

Mrs. Hamilton said the weather, which has been temperate, was probably not to blame. ''It is very puzzling,'' she said.

''Some owners believe it was due to airplanes breaking down the sound barrier which sent (the pigeons) off course,'' she said. ''Others blame microwave ovens, radar and radio signals.

''If aerosols can punch a hole in the atmosphere, these ideas may not be as silly as they first sound,'' she said.

Mrs. Hamilton said many of the pigeon owners in Sheffield, which entered 600 birds in the race, were holding vigils from dawn to dusk in the hope of spotting their birds.

The British Broadcasting Corp. reported Friday that at least one bird had been found among the street pigeons that congregate in London's Trafalgar Square.