Three men have been charged by police in Britain over the July 7 bomb attacks which killed 52 innocent people and injured over 700 in 2005.

Waheed Ali, Sadeer Saleem and Mohammed Shakil are the first to be charged in the case involving suicide bombings in London's transport system.

Peter Clarke, head of Scotland Yard's Counter Terrorism Command, was quoted by Sky News on Thursday as saying that investigations into the bombings had now reached "a new stage" following 21 months of "intense investigation".

The three men have been in custody since March 22.

Two were arrested at Manchester Airport when they were due to catch a flight to Pakistan while a third was arrested in Leeds shortly afterwards.

They are accused of conspiring with the July 7 bombers between the November 1, 2004 and June 29, 2005 to cause explosions on the London transport system or at tourist attractions in the city.

The three men are due to appear at the City of Westminster Magistrates' Court in London on Saturday.