From the Independent's story today:

The Home Office is suspected of being behind discredited media reports on the eve of the Queen's Speech that an al-Qa'ida plot to fly planes into London skyscrapers had been foiled.

[...]

Anti-terrorism sources denied the claim. One said: "To say we were surprised at the report is an understatement - this is the first we have heard of a plot like this."

[...]

Details of the story, which they agreed to run at the same time, are believed to have been provided by a Home Office source several days ago.

In other words the Government deliberately spreads lies with the intention of frightening people for its own political ends.