Jo's Cervical Cancer Trust said a national drop in cervical screening combined with "unprecedented strain" on the health service could lead to more women being diagnosed with preventable cancers.
The charity's warning came as NHS England data showed nearly a third of the 82,053 women eligible for cervical screening in Milton Keynes during 2020-21 did not attend an appointment.
Intended to detect abnormalities within the cervix, routine 'smear tests' are offered to women between the ages of 25 and 64 in an effort to prevent cervical cancer.
Around 70% of eligible women in England were tested during the pandemic, but coverage dropped by 2% compared to 2019-20.
In Milton Keynes, 69% of those eligible were screened – down from 71% the year before.
It means an estimated 25,700 women in the area missed out on the potentially life-saving programme during the pandemic, when invites to screenings were temporarily suspended and appointments delayed.
Many women were also put off attending their screenings due to concerns about their risk of catching coronavirus during the appointment, according to research from Jo's Cervical Cancer Trust.
Samantha Dixon, the charity's chief executive, said the national drop in screenings was not unexpected in light of the pandemic but said it remained a worry.
She said: "Our health service is under unprecedented strain at the moment and facing a long winter.
"We cannot afford to let coverage slip further.
"It will only lead to even more cancers that could have been prevented.
"In some areas coverage is lower than one in two and that should be ringing alarm bells."
Cervical screenings look for changes in the cells of the cervix which could develop into cancer.
During the screenings, a soft brush is used to collect a small sample of cells which is then tested for any abnormalities.
Women aged 49 and under are invited for tests every three years while those older receive invites every five years.
Uptake in Milton Keynes during 2020-21 was higher for older women, with 73% of those aged 50 to 64 receiving a smear test, compared to 67% of women between 25 and 49.