More than 9,000 patients were treated on mixed-sex NHS wards in the 12 months to the end of July, figures show.
NHS England data, analysed by the Liberal Democrats, showed that the number of patients suffering the "indignity" of staying in mixed-sex wards rose to 908 last month.
Overall, 9,004 patients stayed on mixed-sex wards between August 2016 and July 2017, close to a 55% increase on the previous year.
The Conservatives had pledged in their 2010 and 2015 general election manifestos to end mixed-sex hospital wards but it was dropped from the policy programme Theresa May took into this year's election.
Liberal Democrat health spokesman Norman Lamb said: "Under Theresa May's watch, we've seen a scandalous rise in the number of patients being forced to stay in mixed-sex wards.
"Men and women shouldn't have to share hospital wards, it's an affront to basic human dignity.
"It's time to end the the continued underfunding of the NHS and care that is putting intolerable pressure on services and leading to these sorts of failures.
"The Conservatives once pledged to end mixed-sex wards, now it seems they've given up.
"The Government must recommit to closing mixed-sex wards instead of turning a blind eye."