The Foreign Office has been a “target of a serious cyber security incident”, according to a public tender document.
The document, which did not outline what happened or who was behind it, said urgent support is required to “support remediation and investigation”.
It added that BAE Systems Applied Intelligence was called in to deal with the incident and received more than £467,000 for its work.
The document said the work required involved “business analyst and technical architect support to analyse an authority cyber security incident”.
The firm’s contract was concluded on 12 January this year, although it has not been confirmed when the incident took place or the extend of the damage.
It is understood that all parts of the Government implement cyber security defences to identify and prevent potential attempts by cyber criminals to gain access to networks.
The tender document, which was published on Friday, said: “The Authority was the target of a serious cyber security incident, details of which cannot be disclosed.
“In response to this incident, urgent support was required to support remediation and investigation.”
A Foreign, Commonwealth & Development Office spokesperson said: “We do not comment on security but have systems in place to detect and defend against potential cyber incidents.”
Additional reporting by Press Association.
