Professor Mark Woolhouse, a member of the Sage scientific advisory group, said a third wave of Covid-19 was "entirely possible".
Asked about the potential scenario on BBC's Andrew Marr show, the epidemiologist said: “That’s entirely possible... a scenario I mentioned earlier does actually include this possibility.
"This is just another demonstration that lockdown doesn’t solve the problem, it defers it. That is why we need some kind of cavalry on the horizon or, alternatively, if we think that vaccine is not going to be available in six months, or 12 months or two years or whenever it may be, that we do need alternatives."
He warned "mass testing" could be one of few alternatives to a vaccine if the country is to avoid a third spike.
The Edinburgh University professor also revealed the Government "basically" did not model any response other than a lockdown in March, "and that is a worry".
It comes after Prime Minister Boris Johnson introduced a raft of new restrictions to tackle the UK's "inevitable" second wave of coronavirus , with surging case rates plunging 17 million Britons into local lockdown .
Turning to the case of Sweden, which has avoided a high cases and death toll despite having no lockdown with minimal restrictions, Prof Woolhouse added: “It is possible to turn an epidemic around without a strictly enforced lockdown.
"What Sweden have emphasised and the epidemiologists emphasised is the importance of sustainability. We are in this for the long-term and the measures have to be ones we can live with."
He added that the UK's initial lockdown had been an "unproven intervention".
“There isn’t a proven scientific basis for any of this. When we introduced lockdown in the first instance that was an unproven intervention," he went on.
"By the time we introduced it in the UK on March 23, only one city in the world had demonstrated that it’s possible to turn this epidemic around using lockdown.
"Because this is a new challenge, we’re having to use interventions that we’ve never tried before - all of this is very much ad hoc interventions [and] we’re learning as we go.”
But Culture Secretary Oliver Dowden insisted there was "definitely science" behind the new 10pm curfew on hospitality venues, which is entering its fourth day.
He told BBC One’s The Andrew Marr Show: “There is definitely science behind it, that’s why we’re requiring people to be seated in pubs and restaurants, so that stops the flow of them to and from the bar.
“We are reducing the closing times to stop people staying later and drinking. And the point about all of this is that everyone has their part to play. If we all play by the rules, we can ensure that there are not further, more draconian restrictions.”
On Saturday, police in London were forced to break up an anti-lockdown protest after demonstrators failed to socially distance. Thousands of people turned out for the rally.