A fourth tier of coronavirus restrictions is expected to be introduced in London and south-east England, sources have told the BBC.
There is also expected to be a tightening of the plans to relax the rules around households gathering during the Christmas period.
The PM is to hold a press conference at 16:00 GMT as concerns grow about the spread of a new variant of the virus.
Boris Johnson hosted a cabinet meeting earlier to discuss what action to take.
Chief medical officer Prof Chris Whitty said government advisers "consider the new strain can spread more quickly".
