Patients were placed on mixed-sex NHS hospital wards nearly 50,000 times last year - a record high.
Charities said the "alarming" increase in breaches of rules to protect patient dignity was "deeply concerning".
The practice was supposed to be banned in 2010 as part of efforts to protect privacy and dignity.
But the latest data from NHS England show there were 49,100 breaches in the year to November - more than double the 19,700 logged in the year before the pandemic.
Over the 12 months ending in November, the number of breaches increased by more than 10 per cent, from 44,379 the previous year.
NHS rules say hospitals must eliminate mixed-sex wards except for in certain circumstances such as when patients choose to share wards with the opposite sex or require highly specialised care.
The single-sex rule applies to sleeping accommodation, which includes any area where patients are admitted on beds or trolleys - even if they do not stay overnight.
Rachel Power, chief executive of the Patients Association, said: "The alarming increase in breaches of mixed-sex accommodation is deeply concerning for patients.
"While we recognise the immense pressures currently facing the NHS, compromises to patient dignity and privacy are unacceptable and should not become the norm.
"The Government and NHS England leaders must urgently work together to prioritise dignified care, ensuring all patients are treated with the respect and privacy they deserve."
The latest 12 months saw more breaches than any other 12-month period with complete data, while November's 3,983 instances were higher than any other November since records began in 2011.
Data were not recorded between March 2020 and September 2021 because of the impact of the pandemic, while a policy to fine trusts £250 per breach of the rules was dropped in April 2021.
It follows warnings that patients are dying in hospital corridors so crowded that nurses cannot reach them.
Harrowing accounts from more than 5,000 nurses lay bare a devastating collapse in care standards.
The report by the Royal College of Nursing found patients dying in hospital corridors, with some undiscovered for hours.
Britain's most senior A&E doctor said the devastating testimonies must become a "watershed moment" for the NHS, saying "the horror of the situation is stark".
A spokesman for NHS England said: "The NHS is committed to offering same-sex accommodation and there is more to do to eliminate unjustified breaches.
"We recognise this is challenging for NHS trusts, especially at times of extreme pressures on services - and ensuring the privacy and dignity of patients remains paramount when same-sex accommodation cannot be provided."