The next two weeks will be "absolutely crucial" in ensuring that England's coronavirus lockdown ends as planned on December 2, a Government scientific adviser has warned.
Professor Susan Michie, a member of the Government's Scientific Advisory Group for Emergencies (Sage), urged the public to resist breaking current rules, to "be in a position" to spend the festive period with loved ones.
She also suggested that the announcement of a potential Covid-19 vaccine could lead to complacency with the measures, adding that the jab will make "no difference" to the current wave.
It comes after documents released by Sage on Friday warned that a return to the tiered system of coronavirus restrictions will see infections rise again.
When asked what should replace current restrictions when lockdown ends, Prof Michie told BBC Radio 4's Today programme: "It's too early to know. I think the next two weeks is going to be absolutely crucial.
"They're going to be a very challenging two weeks, partly because of the weather, partly because, I think, the promise of a vaccine may be making people feel complacent.
"But the vaccine is very unlikely to come in until the end of the year or beginning of next year and that's going to make no difference to the current second wave.
"So I think for the next two weeks, everybody has to really get all their resolve together."
Prof Michie, a behavioural scientist at University College London, advised the public to "really pay attention to resisting any urges to break the rules" on social distancing and visiting other households.
"Because that will maximise the chance that in two weeks' time, on December 2, we're in a position where actually we don't have to continue the lockdown," she added.
"And better still, what everybody wants, is to be in a position where they can spend the Christmas and winter holiday times with loved ones."
When asked if this meant the gains during lockdown would be lost, Prof Michie said she was "quite hopeful" after tough measures in Wales and Northern Ireland brought transmission rates down.
Newly-released documents, written the day before the second national lockdown was imposed, show a consensus statement prepared by a modelling subgroup of Sage raised concerns about returning to the tier system.
Modelling found that if the lockdown is "well-adhered to", it is likely to reduce the reproduction number to less than 1, with hospital admissions and deaths expected to fall until at least the second week of December.
But the document, dated November 4, added: "If England returns to the same application of the tiering system in place before November 5, then transmission will return to the same rate of increase as today."
Other documents from late October state that any hopes of families gathering at Christmas will also be dependent on the R value staying below 1 for "some time".
On Friday, Sage said that the R rate for the UK has fallen to 1-1.2, with experts believing it is already below 1 in some places.
It is hoped that R will drop in more places next week or the week after, as people remain under lockdown restrictions.
Elsewhere on Friday, the Government said a further 376 people had died within 28 days of testing positive for Covid-19.
As of 9am on Friday, there had also been a further 27,301 lab-confirmed cases of coronavirus in the UK, compared to 33,470 on Thursday.