The number of patients placed in mixed-sex wards in England has risen by almost 70% in the last year.
Statistics from NHS England show 7,163 patients were put in the wards in 2016, up from 4,248 in 2015.
In April 2011, fines were introduced to try to eradicate the problem, leading to a fall from thousands per month to hundreds.
The Patients Association said the rise was "concerning" and that the wards should be scrapped.
Its chief executive Katherine Murphy said single-sex wards are an "important component of preserving patient dignity" in hospitals.
"It is really concerning that over the past year there has been a sharp increase in the number of patients being placed on mixed-sex wards, as a result of mounting hospital pressures," she said.
"Protecting standards of patient safety must remain at the very heart of the NHS and eliminating mixed-sex hospital accommodation is central part of this."
Since April 2011, trusts have been fined £250 when a patient is placed on a mixed-sex ward and since the previous year, statistics have been published on how many breaches of the mixed-sex accommodation rules there have been each month.
In December 2010, there were 11,802 breaches and this dropped to 907 in December 2011 and 314 in the same month in 2012.
In 2014 there was a total of 2,585 patients placed on mixed-sex wards – a figure which increased by 177% in 2016.
Dr Jacqueline McKenna, director of nursing and professional leadership at NHS Improvement, said in some situations there was "no alternative".
"This includes situations where patients need urgent, highly specialised or high-tech care such as in intensive care units. When making this decision, staff must make sure that it is in the interests of all patients affected and work to move patients into same-sex accommodation as quickly as possible.
"A small number of patients (especially children and young people) will actively choose to share with others of the same age or clinical condition, rather than gender."
She added: "The aim of the NHS is to reduce mixed-sex accommodation and ensure privacy and dignity for all patients, but sometimes the need to treat has to take priority, particularly when hospitals are exceptionally busy."
A breach in rules is counted as soon as a patient is placed in a mixed-sex ward and does not depend on how long they stay there.
Single-sex accommodation means patients sharing sleeping, bathroom and toilet facilities only with people of the same sex.
Wards are allowed to be segregated into distinct bays as long as they have separate facilities.