The National Health Service bought back an entire private hospital yesterday, leaving ministers to shrug off suggestions that the taxpayer was "propping up" the independent health sector.

The unprecedented £27.5m purchase means the revamped Heart Hospital, in central London, has become a part of the NHS and should reduce waiting lists because it will double the number of cardiac operations performed in the capital.

The decision has made union officials uneasy because the hospital will continue to treat about 600 private patients a year despite its new public role.

In addition, 162 staff at the hospital transferring to the NHS will retain their superior private sector salaries ­ creating a two-tier pay structure.

However, ministers said the deal, which was sealed on Monday, was a "golden opportunity" to improve capacity for heart surgery in London.

The hospital's former owner, Singapore-based Parkway Group Healthcare, decided to sell the 95-bed unit because it was struggling to attract enough patients.

Parkway paid £60m to buy the Heart Hospital from the NHS in 1991. The hospital's chairman Sir Richard Needham said the NHS had, therefore, doubled its money and had a "tip" of a hospital transformed into a state-of-the-art service in the meantime.

But Geoff Martin, London convenor of Unison, claimed the purchase was a "bail-out" rather than a renationalisation.