Charity Breast Cancer Now estimates there has been a 50% rise in the number of women in the UK who have not had vital breast screening since services restarted last summer.
Finding and treating these women "must be a priority", the charity warned, as "these women do not have time to wait”.
Overall, Breast Cancer Now believes almost 1.5 million fewer women had breast screening between March 2020 and May 2021 when compared with pre-pandemic levels.
Disruption to NHS services has been caused by a variety of factors, including screening being paused at the height of the pandemic and fewer women being referred to specialists with possible symptoms.
This combination means that almost 12,000 people could be living with breast cancer without knowing it.
Breast Cancer Now said that for NHS England to meet its March 2022 target of addressing the shortfall in people starting cancer treatment, an extra 10,000 people would need to have started treatment for breast cancer between May 2021 and March 2022.
However, it said it is unclear how an already overstretched NHS workforce will be able to meet this demand.
The warning comes as the Royal College of Radiologists joined the charity in saying breast imaging and treatment services were “massively under-resourced even before the pandemic hit”.
It said breast screening teams are now trying to fit two years’ worth of appointments into one year.
Baroness Delyth Morgan, chief executive of Breast Cancer Now, said: “A year ago we reported with concern that almost one million women had potentially missed breast screening due to services being paused in the first wave of the pandemic.
“Unfortunately, despite our hardworking NHS staff, screening services running at reduced capacity means that now 1.5 million fewer women have been screened – a staggering 50% increase since services restarted.
“Women with breast cancer are continuing to pay the price due to the impact of the pandemic and, in the worst cases, delayed diagnoses could mean that some women die of this devastating disease.
“Quickly finding and treating those with undiagnosed breast cancer must be a priority, and governments across the UK must urgently ensure there is sufficient investment to do this – these women do not have time to wait”.
She said there needed to be “urgent investment in the chronically understaffed imaging and diagnostic workforce” to tackle the backlog and help ensure women with possible symptoms see a specialist quickly.
Dr Jeanette Dickson, president of the Royal College of Radiologists, said: “Breast services, including screening, are working flat out to make sure patients are seen as quickly as possible, and we cannot urge people enough: if you have any worrying symptoms, please seek help from your GP. If you are given a screening appointment, please take it”.
She added that the priority of the government in the short term to tackle the backlog would be to invest in scanners and IT connectivity and push through stalled service improvements.
“But ultimately, we cannot get away from the need to invest in people. The NHS needs more imaging and oncology staff to ensure future breast cancer patients get the care they deserve”, Dr Dickson said.
An NHS England spokeswoman said: “The pandemic has inevitably meant that some patient services have been disrupted, which is why the NHS in England is investing more than £70 million in additional funding for screening capacity, so people can get the checks they need.
“Extra weekend and evening clinics will help every woman who needs a screen to access one, and the NHS in England has also been offering open invitations to get screened, so if you’re invited for a screening appointment, please come forward and attend”.