© Owen HumphreysCustomers inside the Mile Castle pub in Newcastle (Owen Humphreys/PA)
There is less than a 50% chance that the June 21 plan for lifting all legal limits on social contact will be delayed, a member of the Scientific Advisory Group for Emergencies (Sage) has said.
Professor Graham Medley, from the London School of Hygiene & Tropical Medicine, offered a degree of optimism about the summer though he stressed the Indian variant meant things were still uncertain.
Business Secretary Kwasi Kwarteng also insisted that the June 21 date for the ending of restrictions in England was likely to be met.
He told Sky News that "people should have common sense, they should use judgment and I think if we act in a reasonable way, there is no reason to suppose that we can't reopen the economy entirely on June 21".
On LBC Radio, Mr Kwarteng added: "We need to be cautious because if we get too carried away and the mutant variant spreads too quickly, that could endanger our ability to open up on June 21."
Speaking in a personal capacity on LBC, Prof Medley said the chance of a delay in June was "well less than 50%" but added "it is uncertain".
Asked later about June 21 and how confident businesses such as theatres could be in reopening, he said: "It's a risk. I think it's better than 50% that we'll go through this next phase without having to close things again rapidly, but we're moving back to the situation we were in 2018 before all this all started when there was a risk that we would have a pandemic, but people weren't factoring that into account.
"Now we are in the middle of this epidemic and so the risks are bigger, but it's impossible to give an accurate number to it."
Prof Medley said he thought "there's really one other wave" of infection to come in the UK, but hopefully "vaccines will hold back the virus and prevent people going to hospital". © Provided by PA Media(PA Graphics)
He said there would be another wave "at some point during the summer, maybe late into September or October, depending on how much people mix - so one of the things we can't do is with the modelling is to basically predict what people will do".
Asked if the opening up this Monday was going too far, Prof Medley said the decision was one for the Government.
He added: "There has always been a risk that if we have another wave of infection between now and the end of the vaccination programme that we will end up with large numbers of people in hospital.
"This new variant does seem to be more transmissible, and so it's just increased that risk a bit."
On whether he would go to a pub or restaurant indoors on Monday, he said: "If it was suitably organised, and it looks OK and I was in an area with low prevalence and the clientele was very old, then I would think 'OK, fair enough, they've all been vaccinated'.
"I think it's about individual risks and people taking that choice, which is different from what the Government has to do, which is to avoid the risk of large numbers of people in hospital again."
Meanwhile, Professor Adam Finn, from the Joint Committee on Vaccination and Immunisation (JCVI), suggested he was hopeful June 21 could go ahead but said unexpected things happened in pandemics.
"That's been the story all the way through, that things don't always turn out the way you expect," he said.
"You lay down plans and then something changes, and I think we're still in a place where that is perfectly likely to happen.
"I really hope that these current concerns around this variant evaporate, that everything goes to plan, but I think we just have to accept the possibility that we're in for another big wave and that we will have to change what we're doing.
"That's not good news for people in businesses I know, but it's the reality. We're faced with real uncertainty here."
Elsewhere, Peter Openshaw, professor of experimental medicine at Imperial College London, who is a member of the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), suggested if people were cautious now, things would look better in the summer.
He told BBC Breakfast: "Some of us are quite happy not to be hugging and kissing many times on the cheek.
"This is a high-risk procedure, I would say in medical terms and I would certainly not be embracing people closely.
"I think you can greet people perfectly well at a distance with a smile and a kind word."
He added: "I think we must be extremely cautious. I think we're all in agreement that this is a moment when we need to be very cautious if we're going to preserve our freedoms going forward into the summer.
"The more cautious we are now, the more likely it is that we're going to be able to open up as we hope to over the summer."
Sir Jeremy Farrar, director of the Wellcome Trust, said vaccination had been enormously successful and it was hoped vaccines would keep people out of hospital.
"I think we will see an increase of cases and infections over the coming weeks as some of the restrictions are lifted, but I think the key question is whether we have decoupled increased transmission and number of people who do get infected from the number of people that get ill and need to go into hospital or with long Covid," he told BBC Radio 4's Today programme.
"If we've decoupled them, then I think the country can cope with a marginal degree of an increase in transmission.
"So that is the key question and, to be honest, we don't know that today and that is why I think a very careful lifting is reasonable, but we may have to reverse that if there is escape from the vaccine."
He added: "I just think we're at this point where we've lifted restrictions, and yet we don't have that full amount of information - I think it is reasonable to lift them today, but I do believe all of us need to be really, really careful."