His agent Dave Davies confirmed the shocking news with a tweet from Bruno’s account.
It is understood Bruno was taken ill over the weekend after an event hosted by comedian Jed Stone in Harrogate on Saturday night.
The 57-year-old has been treated at The Horton General Hospital in Banbury and now faces a “few weeks” of rest in order to make a full recovery.
A tweet from his account said: “Hello its Dave Davies, Franks agent, here just to advise as we are getting calls.
“Frank has contracted Pneumonia.
“We must say a big thank you to the staff of The Horton General hospital, in Banbury who have treated him.
“He now needs a few weeks rest.”
Hundreds of well-wishers have already responded to the tweet to pay tribute to the former champion and send him their thoughts for his recovery.
Bruno is widely regarded as one of Britain’s greatest ever boxers after a glittering heavyweight career in which he won 40 of his 45 fights and registered 38 knockouts.
He won the WBC heavyweight title in his fourth world title attempt, outpointing Oliver McCall in 1995 before an eye injury suffered in his second defeat to Mike Tyson brought his career to an abrupt end.
He was involved in numerous huge fights including an all-British dust up with the legendary Lennox Lewis for the WBC crown.
The charismatic Londoner, 57, has fought a brave battle against the stigma of mental health after well-publicised personal battles in the past which saw him sectioned under the Mental Health Act in 2003 and twice in 2012.
Bruno became a grandfather for the first time last year and revealed he has gone more than three years without medication in his fight with bipolar.
He was awarded an honorary doctorate at Bournemouth University as recognition for his fine work last year.
After receiving the award, he said: “This honour, I would like to think, it’s not only for my achievements in boxing but also in recognition of the work and the constant battle I’m trying to fight on behalf of the sufferers and carers of sufferers who have experienced this terrible strain of mental ill health.
“I am honoured more than I can truly express.”
He is now set for a period of down time as he rests and recovers from his illness after being treated in hospital.