Boris Johnson was under pressure from Tory MPs to set out plans to ease Covid-19 restrictions and deliver a vision for living with the virus.
Cabinet minister Nadhim Zahawi ruled out universal free lateral flow tests being scaled back yet, but suggested the reduction of isolation times could be used to ease staffing issues.
Chancellor Rishi Sunak was also among ministers keen on the economic benefits of reducing the period from seven to five days, according to the Daily Telegraph.
Mr Harper warned Mr Johnson he could suffer an even greater rebellion than when he introduced his Plan B measures if he tries to extend them later this month.
The Prime Minister, however, was understood to be unlikely to set out further plans while cases rates remain so high and the NHS strains under significant pressures.
With an extra 141,472 lab-confirmed cases announced on Sunday, the figure fell for the fifth day in a row. However, this should be treated with caution as reports often drop on the weekend.
Mr Zahawi, the Education Secretary, insisted that lateral flow tests will remain free for the time being after the Sunday Times reported their provision could be massively reduced.
But he did not indicate how long they will remain free for and he set out an ambition for the UK to be “one of the first major economies to demonstrate to the world how you transition from pandemic to endemic”, or regularly occurring.
“But I would absolutely be driven by advice from the experts, the scientists, on whether we should move to five days from seven days. What you don’t want is to create the wrong outcome by higher levels of infection.”
Mr Harper, the chair of the lockdown-sceptic Covid Recovery Group, warned the rebellion if the Prime Minister tries to extend Plan B beyond January 26 could be even larger than the 100 Conservatives who defied him over their introduction last month.
“I think there will be even more people against it,” he said in an interview with the Financial Times. “I think the intellectual argument now is even weaker.”
Dr Clive Dix, former chairman of Britain’s vaccine taskforce, said mass testing and vaccination should end for all but the most vulnerable after the booster campaign is over.
“It’s pointless trying to stop infection with it, which is sort of what mass vaccination is all about, because it’s not doing it. We’re seeing a lot of infection,” he told Channel 4 News.
Dr Dix added: “I think that’s a little bit controversial but let’s look at a couple of months’ time, we shouldn’t be mass testing. I think mass testing doesn’t help anybody.
“I think we need to get to the point where if we have a young person who gets Covid, having been vaccinated, we know they’ve got levels of protection, but just like if they’ve got very bad cold or flu, they stay at home… and when they get better they go back to work.”