Warnings over a possible "twindemic" of COVID-19 and flu have been issued each winter since the start of the coronavirus pandemic in early 2020, but COVID restrictions that limited social contact have meant flu levels stayed low, Reuters reported.
However, the government ended coronavirus restrictions earlier this year, meaning that social contact rates have returned to near pre-pandemic norms while immunity to flu is relatively low.
The UK Health Security Agency (UKHSA) said that given the risk it was important those eligible took up vaccines against COVID and flu.
"There are strong indications we could be facing the threat of widely circulating flu, lower levels of natural immunity due to less exposure over the last three winters and an increase in COVID-19 circulating," said Susan Hopkins, Chief Medical Advisor at UKHSA.
After falling over the summer, there are signs that COVID-19 cases and hospitalisations are starting to tick up, and Hopkins said lots of variants currently circulating could evade the immune response to some extent.
Around 33 million people are eligible for a free flu vaccine this year, and 26 million are eligible for a COVID-19 booster. The elderly and clinically vulnerable are eligible for both, and young children can get flu shots.
If concerns about a so-called twindemic materialise, it will heighten pressure on Britain's already stretched hospitals, which are bidding to catch up with procedures delayed during the pandemic and struggling with a staffing crisis.
On Tuesday, Labor Party leader Keir Starmer said he was really worried with how many lives were at risk this winter given the strain on the NHS.