TOP-secret planning documents of MI6’s HQ have gone missing in a shocking security gaffe.

More than 100 papers, detailing layout and security measures, vanished from the building, which has featured in James Bond films like Skyfall.

2 Building giants Balfour Beatty have been sacked from renovating MI6’s HQ after the papers disappeared Credit: Alamy

One source said last night: “To lose such sensitive documents was grossly irresponsible.”

Furious chiefs investigated before terminating Balfour Beatty’s contract on the multi-million pound refurbishment project.

It comes after the new Tory Government last week announced proposals to strengthen UK security against foreign spies and hackers.

The secret documents were ­supposed to have been kept in a secure room during work at the HQ of the UK’s Secret Intelligence Service (SIS) — better known as MI6.

Access was said to have been limited to a small number of supervisors overseeing the refurb.

But the alarm was raised a fortnight ago when stacks of the documents vanished at the building, which has featured in several James Bond films and was “destroyed” in Spectre.

One source said: “The whole building went into lockdown and all the construction workers were kept in isolation.

“To lose such sensitive documents was grossly irresponsible.

“They showed the layout of the building and specifically where alarms and other security measures were. The documents would be gold dust to any enemy agents or terrorists.”

'LOCKDOWN'

The Sun understands senior execs from Balfour Beatty were brought in to explain the breach.

The papers were not classified, but were deemed to be highly ­sensitive due to their content.

Many were later found still inside the building at Vauxhall Cross, South London.

Some remain missing although spymasters are said to be confident they are not in enemy hands.

However, a decision was taken to terminate Balfour Beatty’s contract.

The source said: “A lot of the work was sub-contracted and there were around 40 workers on the job. Security was supposed to be incredibly tight. Workers and ­vehicles delivering materials

were thoroughly searched. The importance of security was drummed into the contractors — but clearly not taken on board.”

M16 is the foreign arm of the UK’s intelligence services. As well as espionage against hostile states, it plays a major role in tackling terrorism and cyber warfare.

Chinese and Russian agents are known to monitor the MI6 building, opened in 1994. The spy threat was highlighted last week when the Government proposed making it a specific criminal

offence.

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­Balfour Beatty is the largest ­construction group in the UK and last year had a £7billion turnover.

The source added: “The decision to terminate their contract would not have been taken lightly. But it was clearly felt they had let the side down.”

Balfour Beatty declined to comment. The Foreign and Commonwealth Office, which is responsible for MI6, said: “We do not comment on intelligence matters.’’

2 MI6's Thames headquarters have featured in James Bond films like Skyfall Credit: Kobal Collection - Rex Features

Spook gaffes IN 2000 a manuscript of former MI5 head Dame Stella Rimington’s memoirs were anonymously handed to The Sun in another security gaffe. We refused to publish extracts and gave it to No 10 instead. A month later an MI5 agent left a briefcase containing secret documents on a train to Dorset. MI6 head Sir John Sawers’ security was put at risk in 2009 when his wife posted family pics online. In 2012 Dame Stella lost her laptop at Heathrow.

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