About 3,000 women in Wales have “undetected breast cancer” because screening services were suspended during lockdown, it has been warned.
Tenovus Cancer Care estimates 30,000 people missed out on mammograms between March and July, and fear a repeat as coronavirus cases rise again.
Claire Williams, 39, was told she might not have survived breast cancer if she had not been treated when she was.
The Welsh Government said it had worked hard to ensure screening can continue.
Breast cancer screening was suspended by NHS Wales in March as the health service was gripped by the pandemic.
The service resumed in August, but Judi Rhys, chief executive of Tenovus Cancer Care, said 30,000 people missed out on a screening appointment during this time.
“About 3,000 women would have been picked up and referred onwards for further checks during that period,” she said.
“That actually means there are 3,000 women in Wales walking around with undetected breast cancer at the moment.”
Ms Rhys said it was a “potentially very serious” situation for those women.
“We do know that if breast cancer is diagnosed at a very early stage then the prognosis is far, far better and the outcomes are far, far better than if it is picked up at a later stage,” she added.
“If people notice anything at all we are urging them to make an appointment with their GP.”
‘I’m grateful I’m here’
Things could have been very different for Claire Williams, from Swansea, who went to see her GP when she found a lump on her breast last year.
She was told she would have to wait nine weeks for further tests on the NHS, but was able to access private healthcare through her employer.
“Within two hours” of her consultation she was told she had breast cancer and would need chemotherapy, a mastectomy and radiotherapy.
“Looking back on the last 18 months and the experience I have had, I am grateful that I am here,” Ms Williams said.
“And I am grateful the children will be growing up with me in their lives.”
Ms Williams said the number of people who had missed out on screening during lockdown was “astounding”.
“It’s scary for the women who might have missed them,” she added.
She called on people to “be familiar with your bodies” and get checked if they were concerned about anything.
Ms Rhys said the health service was in the process of clearing a “backlog” of 30,000 screening appointments.
But she said it was vital that people continued to be screened, even in the event of another lockdown.
“We are worried that with Covid on the rise again that people are either too scared to come forward or that some services may need to be paused again,” she added.
A spokeswoman for the Welsh Government said it had taken action to ensure staff and patients were safe from coronavirus, and encouraged women to attend their screening appointments.
“The NHS has undertaken extensive work to ensure as much cancer care as possible can continue during the pandemic,” she added.