A source on background has confirmed to CNN that sensitive documents that had been reported missing by The Sun newspaper were not UK government papers, but were a construction firm's documents.

On Friday, The Sun reported that "sensitive" documents detailing the "layout and security measures" of MI6's London headquarters have gone missing while construction giant Balfour Beatty carried out refurbishment work.

The missing documents were supposed to be held in a secure room by the construction firm while the renovations took place at the headquarters of the UK's Secret Intelligence Service -- better known as MI6 -- the UK's equivalent to the CIA.

Although not classified, the papers were deemed to be highly sensitive due to their content.

"To lose such sensitive documents was grossly irresponsible. The documents would be gold dust to any enemy agents or terrorists," The Sun reported an unnamed source as saying.

Balfour Beatty's contract to refurbish the building at Vauxhall Cross, south London, was terminated after the papers disappeared, The Sun reported.

The UK's Foreign Office said it would not comment on intelligence matters.

CNN has reached out to Balfour Beatty for comment. (CNN)

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