The UK's vaccine advisers are understood to have approved the rollout of Covid booster jabs to people under 50, with a minister saying an announcement was due on Monday.
The Joint Committee on Vaccination and Immunisation (JCVI) was scheduled to set out the next steps in the booster programme "later today", Oliver Dowden, the Conservative party co-chair, told Sky News.
Dowden said: "It's up to them but I would hope that we would see a further expansion of the booster rollout. But we'll wait for their announcement."
While Dowden did not set out what changes were expected, it is understood that the JCVI has already decided that the scheme for boosters - a third injection to top up potentially waning immunity - should be extended to younger age groups.
Those currently eligible for a booster, which is usually given six months after the second dose, include those aged 50 or over, or anyone younger seen as clinically very vulnerable to Covid. Others who are eligible include frontline health and care workers, or those who care for someone at high risk from Covid.
There is a separate programme of third injections for people who have a compromised immune system, for whom vaccines are often less effective.
While the broad decision to extend the booster programme is made by the JCVI, it remains unknown which other groups may be offered a booster. The committee is expected to meet on Monday morning to discuss further issues.
Ministers see the extension of boosters as a key element in efforts to mitigate the impact of Covid over the winter, with the number of booster jabs now picking up after as slow start. So far, 12m boosters had been given, with 2m in the last week, Dowden said.
Among efforts to speed up the process, some people are now being invited for boosters five months after their second jab, rather than the previous six.
While Covid infection rates have fallen during November, they are now on the rise again, while more than 1,000 deaths connected to the virus were recorded in the week to Sunday.
Dowden said the likelihood of new Covid restrictions being needed during the winter was "in our hands". He told Sky: "If you get the booster when the call comes that is the biggest wall of defence that we have against Covid. I am confident that if we stick the course, people take the boosters when they are asked to do so, that vaccine wall will hold up and we will be able to have a decent Christmas this year."
Among other decisions yet to be announced is when people aged 16 and 17, who have already been offered a first Covid vaccination, will be offered a second. The JCVI is understood to have approved this in outline last month, but nothing has yet been announced formally.
The JCVI decided in favour of first jabs for the age group in early August, saying at the time it was likely that second shots would begin 12 weeks after the first dose. But there have been no updates since, with some parents reporting that GPs were telling teenagers they would need to wait until they were 18 for their second dose.