This has concerned GPs, who are forced by Choose and Book to offer patients a choice of private hospital if available.
In some instances, block contracts mean that private contractors are paid higher tariffs per procedure than the NHS.
Last month, prime minister Tony Blair said that by 2008 he wanted to see about 40 per cent of acute operations conducted by the private sector to be carried out for the NHS.
Meanwhile, the NHS is struggling with deficits of almost £1 billion and the Conservative party estimates that up to 20,000 jobs might be cut by NHS trusts in the next year, putting additional pressure on primary care.
According to BUPA's annual report: 'NHS patients now represent about 14 per cent of BUPA Hospitals' business. Treating more NHS patients forms part of a wider growth strategy, enabling BUPA to increase utilisation of hospital beds and theatres. This in turn means healthcare can be made more affordable to BUPA Hospitals' private patients.'
BUPA is reporting profits of £350 million for 2005, 29 per cent up on 2004, with its hospitals contributing £49.5 million.
Clare Hollingsworth, managing director of BUPA Hospitals, said although they were 'pleased to play a part in cutting NHS waiting lists', commitment must be with the privately insured.
East London GP Dr Kambiz Boomla said: 'There are NHS deficits of £1 billion and it is spending £3 or £4 billion on the independent sector. They are a large part of why deficits have arisen. These are racking up NHS costs, with a profit element going to shareholders.'
He warned GPs to be wary of private firms coming into primary care. 'They will not need to operate at a profit,' he said, 'but will want a foot in the door so they can use practice-based commissioning budgets to control both sides of the equation.'
NHS Alliance commissioning lead Dr David Jenner agreed, saying: 'Many companies have expressed an interest in running surgeries and enhanced services. If they have a successful clinic in the area, that will be easier.'
BUPA would not confirm whether it had plans to move into primary care.
Other private providers have yet to release details of their 2005 profits.
But in their 2004 report, Nuffield Hospitals and General Healthcare Group acknowledged that the NHS was an important and growing customer.