The documents, most of which were recovered inside the building, held sensitive information on the layout, including entry and exit points.

Balfour Beatty, the company working on the refurbishment at the headquarters in Vauxhall, is reportedly no longer working on the project.

The Foreign Office said it did not comment on intelligence matters.

The documents, which went missing a few weeks ago, were produced and owned by Balfour Beatty and designed to be used for the refurbishment.

The contractor kept the plans on the site at Vauxhall Cross in a secure location.

BBC security correspondent Gordon Corera said the missing plans were not classified or intelligence documents, but the pages did hold sensitive details.

Most, but not all, of the documents were recovered inside the building after it was noticed they were missing, he said.

Balfour Beatty said it could not comment because of sensitivities.

The incident, first reported by the Sun newspaper, is reportedly a result of carelessness, rather than any hostile activity.

Balfour Beatty mainly carries out work in the UK, US and Hong Kong and has 26,000 employees worldwide, according to the company's website.

The BBC reported earlier this month that Balfour Beatty's order book for 2019 was expected to be more than £14bn - "significantly higher" than 2018's £12.6bn book.