The News understands incident at the Cosham hospital is in response to rising numbers of Covid patients in hospital, continuing pressure on A&E and limited bed capacity.
Latest public NHS figures show there were 85 patients with Covid at QA Hospital as of last Tuesday.
Between December 2 and 6 the hospital had no available general and acute beds – something medical director Dr John Knighton previously warned about.
Last month he told The News the hospital was ‘operating absolutely at the margins of capacity to meet the demand’.
On one day this month – December 5 – there were no critical care beds available at all, the same NHS figures show.
Bed capacity has been hit by medics needing to split Covid-positive patients away from those who do not have the virus.
And it comes at a time when Portsmouth has seen its highest peak in its Covid rate.
There were 269 new cases in Portsmouth on December 6 alone.
QA bosses have previously urged people to get a jab, and choose the right place to get treatment.
Portsmouth City Council leader Gerald Vernon-Jackson said: ‘I think there’s a real chance the NHS will collapse this winter.
‘This is the most serious that I’ve ever seen it. It’s incredibly worrying for everybody so there are pressures in lots of different directions.
‘It’s very difficult for people to see a GP, and I know people are getting very worried about that.
‘People are going to St Mary’s and the waiting times are hours and hours. People go to A&E and people are waiting for hours.’
Councillor Vernon-Jackson said his husband spent three hours in an ambulance outside QA’s A&E department before being admitted.
‘The staff are wonderful, working ludicrously hard,’ he said.
‘People are working incredibly hard and I’m sure everyone is grateful to them – but the staff shortages are frightening.’
He said he was told of NHS workers asked to cancel holiday to work instead.
‘Things are very serious,’ he said. ‘The thing we can all do is to go and get our boosters.
‘If you’ve got your booster your chances of getting seriously ill from Covid are substantially reduced.’