Boris Johnson must be prepared to "move fast" to impose "Plan B" measures such as masks and vaccine passports if Covid infections spiral, the Government's top scientist has said.
Sir Patrick Vallance said the UK was still in a "very uncertain phase" of the pandemic - and the Government needed to be ready to act quickly if the situation worsens.
The Chief Scientific Adviser said Plan B measures needed to remain on the table and ministers need to "push beyond natural reluctance" to impose new Covid curbs.
The Government's Covid Winter plan contains a list of contingency measures if the virus begins to surge, known as plan B.
These include making face masks mandatory in some settings, vaccine passports and ordering people to work from home.
Minutes from a SAGE meeting on October 14 emerged last week, which show Government scientific advisers urged the PM to trigger Plan B ahead of the winter
But the Government has been reluctant to act, with ministers repeatedly saying that the data doesn't point to a need for more restrictions.
The UK's Covid death toll topped 200 for the second day in a row on Wednesday for the first time in seven months.
A further 43,941 cases were recorded that day - an increase on 40,954 infections on Tuesday but a 10.5% dip on this time last week when there 49,139 official cases.
"There is considerable uncertainty into which direction this goes," Sir Patrick told the Today programme.
"It's wrong to think that the build up of immunity is an all or nothing - it's a sort of protective barrier that will reduce the spread of the virus so we need to monitor this carefully over the next weeks and months.
He added: "You need to absolutely be prepared (for plan B) and as soon as you start thinking 'am I, or am I not going to do this? It looks close' is the time you need to push beyond your natural reluctance to do it and do it.
"This is obviously something the government will have to consider carefully but we need to be ready to move fast if that occurs."
Predictions on the spread of the virus hinge on how quickly immunity wanes and also how people behave, which are both tricky to predict, Sir Patrick said.
The rollout of the booster programme has ramped up after a sluggish start in a boost to hopes for immunity levels, with more than half of eligible people getting jabbed.
Currently, boosters are being offered to people over 50 or younger adults with certain health conditions which make them vulnerable to coronavirus.
Downing Street said it was prepared to act to curb Covid rates if necessary but there was no present need to move to Plan B.
The PM's spokesman said: "One of the reasons we set out our Autumn/Winter plan including setting out what Plan B would mean... was so that we are able to act swiftly as required.
"I again emphasise that we don't see anything in the statistics or the epidemiology to suggest we need to move to Plan B.
"But as a responsible Government we are making sure we are ready to act should it become needed."
It comes after a leaked document claimed Plan B could cost more than £800m a week.
A separate analysis seen by the Telegraph claimed that vaccine passports could cost businesses £345m a month or £2.1bn over six months.
