The NHS would have to be rebuilt if the aspiration of single-sex wards were to be achieved, a health minister says.
Lord Darzi believes single-sex wards are impossible
But Lord Darzi said the government was committed to single-sex accommodation whereby wards are divided into male and female bays by fixed partitions.
Labour promised to end mixed-sex accommodation in England by 2002, but that has still not been met.
Campaigners have called for single-sex wards, saying fixed partitions do not give patients enough privacy.
Lord Darzi, a practising surgeon who is currently conducting a review of the NHS in England, told the House of Lords single-sex accommodation should be the "norm".
He said the goal was never to create single-sex wards as this was not achievable.
"The only way we're going to have single-sex wards within the NHS is to build the whole of the NHS into single rooms.
"That is an aspiration that cannot be met."
But his claim drew an angry response from patient groups.
Katherine Murphy, from the Patients Association, said: "We want to see entirely single sex wards, not accommodation.
"Otherwise patients still have to share bathroom and toilet facilities with members of the opposite sex, and will have members of the opposite sex walking past them on their way to use these facilities."
Kate Jopling, of the charity Help the Aged, said: "Dignity in care should be paramount, and privacy goes hand in hand with this.
"Sharing mixed sex wards remains an ongoing concern for many older people who may find the experience distressing and an inappropriate infringement of their privacy, and therefore dignity.
"Singe sex accommodation is a step in the right direction, but this usually means men and women sharing washing and toilet facilities, a far from ideal situation."
A Department of Health spokesman said Lord Darzi's comments were "fully in line" with the government's "long-standing commitment on mixed-sex accommodation".
He said: "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 and admit will take priority over complete segregation.
"The NHS will not turn patients away because the "right sex" bed is not immediately available."
The government's stance has been criticised by the Conservatives.
Shadow health secretary Andrew Lansley said the government had "promised to abolish mixed-sex wards" prior to the 1997 and 2001 general elections, but "now, they say it's all too difficult".
He said: "The reality is that one in five patients go into mixed-sex accommodation. Far too many do not get the privacy and dignity they deserve.
"The government made a promise but now there is a climb-down. Once again, Labour promises, no delivery, and patients are being let down."
Liberal Democrat health spokesman Norman Lamb said the proposed arrangements were not good enough.
"The separation of a curtain, for example, is simply not adequate and many patients feel that this is demeaning," he said.
"And I think patients feel particularly strongly that the commitment should have been met."
It comes as the NHS is still struggling to ensure mixed-sex accommodation is eradicated.
Ministers were insisting as recently as November 2006 that 99% of patients were being seen in single-sex accommodation.
But a report by chief nursing officer Christine Beasley last year - commissioned after patient surveys cast doubt on the claims of ministers - showed 15% of hospitals needed more help achieving this.