British spying chief Alex Allan, the chairman of the Joint Intelligence Committee, has regained consciousness having been in a coma for 10 days.
A Cabinet Office spokesman said there were "positive signs of recovery" in Mr Allan, 57, who has been in intensive care since collapsing at home.
He is expected to remain in hospital for the immediate future while he "regains his strength".
The spokesman said: "His family are delighted with his progress."
Mr Allan collapsed at his home on 30 June.
Government sources say there is no sign of foul play.
Mr Allan's committee collates information from MI5, MI6 and GCHQ and briefs the prime minister, ministers and officials on intelligence assessments on issues such as security, defence and foreign affairs.
The deputy chairman takes over in the absence of the chairman but Cabinet Office policy is not to publicly name the person.