A top construction firm was sacked from its refurbishment of the MI6 headquarters after a major security breach, it was reported last night.

Balfour Beatty was axed from the project after more than 100 blueprints outlining layouts and alarm systems went missing from the home of the Secret Intelligence Service.

The building, at Vauxhall Cross in south London, is famed for its appearance in several James Bond films.

Access to the sensitive papers was limited to supervisors and there were approximately 40 staff sub-contracted to the project, The Sun reported.

So serious was the security lapse that when the alarm was raised staff workers were not allowed to leave.

Balfour Beatty was axed from refurbishing the MI6 headquarters in Londond (pictured) after 100 blueprints outlining layouts and alarm systems went missing from the home of the Secret Intelligence Service

A 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.' The documents would be gold dust to any enemy agents or terrorists,' they added.

Documents included 'where alarms and other security measures were', it was claimed.

Neither the Foreign and Commonwealth Office, under which MI6 operates as the foreign arm of British intelligence, nor Balfour Beatty were available for comment last night.

An FCO spokesman told The Sun: 'We do not comment on intelligence matters.'

Bucking the construction industry's post-Brexit gloom, Balfour Beatty enjoyed increased profits last year as they rose 5.1pc to £123m in 2018

Though many of the missing papers were found inside the building after the alarm was raised, Balfour Beatty executives were reportedly told the firm's multi-million pound contract had been terminated.

Only last week the Government put forward fresh proposals to bolster national security.

Citizens who pass sensitive information to hostile states could be tried for treason under laws being considered by ministers.

Prime Minister Boris Johnson pledged to update offences so they 'deal more effectively with the espionage threat' in a briefing document.

Bucking the construction industry's post-Brexit gloom, Balfour Beatty enjoyed increased profits last year as they rose 5.1pc to £123m in 2018.