Doctors running NHS intensive care units have expressed growing concern that private insurers are refusing to pick up the tab for patients transferred from the private sector.

The Intensive Care Society is meeting private healthcare provider BUPA later this month to discuss the problem. The issue has also been raised by the Commons health select committee, which is holding an inquiry into the regulation of private healthcare.

Patients staying in private hospitals without intensive care facilities rely on the NHS if things go wrong. Although private hospitals have to make arrangements for this care as part of their registration process, the question of who pays is a grey area.

'When things do not work out, they suddenly expect the local NHS to sort them out,' said Dr Carl Waldmann, director of the intensive care unit at Reading Hospital and a member of the Intensive Care Council.

He claimed his unit had dealt with at least 10 patients from the private sector over the past year. Sometimes insurers refuse to pay.

'With one of the patients we were owed something like£16,000. In the end something has been paid, but most hospitals will just give up because of the bureaucracy.'

Dr Lee Baldwin, who works in intensive care at Kent and Sussex Hospital, Tunbridge Wells, found himself at the centre of a battle when he charged his own medical fees to a private patient transferred from a BUPA hospital. The patient paid the bill, assuming the insurers would cover it, but they refused.

'If you are doing enough major surgery, particularly abdominal, you are going to have a certain amount of patients needing intensive care,' said Dr Baldwin. 'If you are making no provision for that, you are getting NHS cover for free.'

But Dr Andrew Vallance-Owen, group medical director of BUPA, disputed the claims. He argued that where a patient suffered something totally unrelated to their operation, the NHS should accept the cost. 'The grey area is when it is a side effect of the operation,' he said. 'Strictly speaking, we would argue that their life was in danger.

'But these cases are very, very rare. I am only aware of 11 transfers in the last year.' In none of these had BUPA insurance paid the NHS for the intensive care given, at an average cost of£550 a day.

Dr Vallance-Owen said BUPA made provision in its insurance payment to hospitals for the cost of intensive care where the hospitals did not have their own units.

Alan Pilgrim, chief executive of Community Hospitals Group, said his hospitals' insurance policy would pick up the cost if there was a clear case of medical negligence leading to the transfer.

Barry Hassell, chief executive of the Independent Healthcare Association, said: 'If I am treated as a private patient, it still doesn't change my rights to use the NHS. I am a tax payer.'

But Dr Peter Nightingale, director of the Withington Hospital intensive care unit in Manchester, said: 'If you are going into a private hospital for a routine surgical operation, the cost of intensive care in the NHS should be covered as well.'