Policing has been increased at Heathrow

After government figures revealed that 11,500 passports were lost in the post between 1999 and 2002, Liberal Democrat spokesman Paul Burstow called for an immediate inquiry.

He said the situation was "scandalous" and posed a serious security risk at a time of heightened fears about a possible terror attack.

Either this is incompetence... or there is a blackmarket trade in passports Paul Burstow, MP

Mr Burstow said an urgent inquiry was needed to establish why so many passports vanished.

"Either there is incompetence on the part of Royal Mail sorting offices when they are delivering passports back to customers, or there is a blackmarket trade in stolen passports," he said.

Mr Burstow said he was concerned the passports could be used fraudulently.

"Basic preventative security measures, like registered mail, must be put in place to prevent identities falling into criminal or terrorist hands," he said.

'Small in comparison'

A Home Office spokeswoman said secure delivery of passports should be in place by the autumn.

"The Government takes very seriously the security and integrity of the UK passport.

"The UK passport service issues between 5.2 million and 5.5 million passports every year, the majority of which are issued via first-class post.

"The number of passports that go missing between dispatch and receipt by applicants is small in comparison," she said.

The figures were passed to Mr Burstow by Home Office Minister Beverley Hughes in a Parliamentary answer.

More than 11,000 passports have been lost since 1999

The passports were lost after being posted to customers by the Passport Agency.

Some 2,359 went missing in 1999, 3,851 in 2000, 2,541 in 2001 and 2,982 in 2002.

The figures do not include passports lost while being handled by the Passport Agency, or those which went astray when posted to the agency.

Suicide bombers

News of the missing passports followed the introduction of new measures to combat a possible terrorist attack in Britain.

It also comes as the Sunday Times reports that MI5 and Scotland Yard are hunting two men in Britain who have been trained as suicide bombers by al-Qaeda.

Detainees at the US prison camp at Guantanamo Bay in Cuba reportedly told British and American authorities the men were members of a specially recruited squad of "martyrs".

Meanwhile concrete barriers have been installed around Parliament to deter would-be suicide bombers and security was stepped up at London airports.

It has also been reported that undercover police squads watching suspected Islamic militants in London have been told they can shoot to kill if they think a suicide attack is being launched.