“Sensitive” plans of MI6’s headquarters have disappeared during refurbishment works – with some still thought to be missing.

More than 100 documents detailing the Vauxhall building’s layout, including entrances and alarms, were lost more than two weeks ago, The Sun reported.

Balfour Beatty, the construction firm carrying out the work, has reportedly been fired. The company – the largest building firm in the UK – said it would not be commenting.

After the documents were discovered missing a fortnight ago, sub-contracted Balfour Beatty workers were kept “in isolation” at the headquarters, according to the paper.

The Independent understands that the documents had been drawn up by Balfour Beatty, rather than the Secret Intelligence Service (MI6).

While some were later recovered within the building, others are believed to remain elusive.

“The whole building went into lockdown and all the construction workers were kept in isolation,” The Sun reported an unnamed source as saying.

“To lose such sensitive documents was grossly irresponsible. The documents would be gold dust to any enemy agents or terrorists.”

The plans, said to show internal floor plans of the building and location of security alarms, are understood to have been held in a secure room to which only around 40 contractors had access.

Officially, they are not believed to have fallen into the hands of hostile powers — but, worst case scenario, the plans are already in the hands of the SVR in Moscow.

The Foreign and Commonwealth Office, which is responsible for MI6, said: “We do not comment on intelligence matters.”

Last week the government announced plans to revamp the Official Secrets Act, which could see the introduction of stricter legislation on the dissemination of information deemed to pertain to national security.

Perhaps it’s time to call on 007 himself, James Bond?