Hundreds of patients suffered harm under County Durham and Darlington NHS Foundation Trust, according to a review of cases dating back to 2023.
The BBC has spoken to multiple women affected by the failings, including one who said she felt "mutilated" and the trust's mistakes were "absolutely unforgivable".
Ahead of agreeing to expand the review to cases dating back to 2015, members of the trust's board offered "sincere" apologies for the failings and harm caused.
At an extraordinary meeting of the board held in Durham, board chair Alison Marshall said: "I would like to offer a sincere apology to all the women and their families who have been affected by the failings in our breast services.
"I can't imagine the impact that has had."
She said they could "not turn back the clock" but would be "open, honest and transparent" about the failings.
Chief nurse Gill Hunt told the board concerns were first raised in 2015 after a "service reconfiguration" and loss of breast surgical trainee posts.
Over the following years patients and bodies such as the Northern Cancer Alliance, Care Quality Commission and NHS England raised multiple issues, she said.
The key "themes" included treatment "not aligned to best practice", excessive surgery and delayed diagnosis, she said.
This breaking news story is being updated and more details will be published shortly. Please refresh the page for the fullest version.
You can receive Breaking News on a smartphone or tablet via the BBC News App. You can also follow @BBCBreaking on X, external to get the latest alerts.
