A rule preventing patients of different sexes from being treated on the same ward was broken multiple times at The Royal Marsden NHS Foundation Trust over a six-month period, new figures reveal.
It comes as the number of breaches across England has soared since the coronavirus pandemic began, with March seeing the second-highest number since 2011-12.
The Patients Association said mixed-sex wards are "an affront to patients' dignity", claiming the stress they cause prohibits a strong recovery.
NHS England data shows a rule preventing different sexes from mixing on wards at the Royal Marsden Trust was broken three times in the six months to March – up from none in the same period the year before.
In the six months to March 2019 before the pandemic, there were no breaches whatsoever.
Nationally, there were almost 4,500 instances where mixed-sex rules were broken in March – the second-highest single month since 2011-12 and more than triple the 1,446 instances recorded in March 2019.
Recording breaches was suspended from March 2020 to September 2021 due to the pandemic, but when logging rule-breaking returned, there were 2,289 occurrences, while every month since this past December has topped 4,000.
Rachel Power, chief executive of the Patients Association, said: "Mixed sex wards are an affront to patients' dignity.
"No patient wants to receive intimate, personal care on a mixed sex ward, and it's the sort of stress that doesn't promote recovery."
At The Royal Marsden NHS Foundation Trust, there were no breaches in March this year or in March 2019, before the coronavirus pandemic.
An NHS spokesperson said: "Offering single-sex accommodation is a requirement under the NHS Standard Contract.
"Trusts across the country are taking action to reduce or eliminate unjustified breaches, which remain rare."
A Department of Health and Social Care spokesperson said: "We have been clear patients should not have to share sleeping accommodation with others of the opposite sex and should have access to segregated bathroom and toilet facilities, and we expect NHS trusts to comply with these measures."