The private sector competes with the NHS and cannot expand without stripping it of staff, resources and patients, according to the King's Fund.

In its spring review of healthcare policy, health systems fellow Justin Keen argues that the sectors are 'in practice interdependent' because the 'supply of doctors and other clinical staff is effectively fixed'.

Attempting to increase the use of the private sector is likely to exacerbate NHS staff shortages. Investing in the NHS could reduce demand for private care, undermining prime minister Tony Blair's pledge to increase total UK healthcare spending to the European average. 'This is the stuff of policy nightmares, where one part of the pledge (boosting the NHS) may serve to undermine another (reaching the EU average), ' Mr Keen says, adding that increasing the pool of available staff would 'take some time'.

Health systems director John Appleby and independent consultant Sean Boyle set out the background to the argument. They say Mr Blair's pledge must be met from an increase in total government expenditure, diverting public money to the NHS or an expansion of the private sector.

If the proportion of government spending on the NHS is to remain constant at 14.4 per cent, total public expenditure would have to rise from 39.7 per cent of GDP to an 'unprecedented' 56 per cent, which is 'likely to be politically and economically unacceptable'.

Leaving public spending at 40 per cent of GDP will mean 'real reductions' in other areas, while trying to meet the commitment through increased spending within the private sector would require 'an immense increase' in private provision.

'Even assuming that NHS funding increased at a rate of 5 per cent per year, private spending would need to increase from£8bn to over£24bn, ' the authors calculate.

Health Care UK Spring 2000. King's Fund bookshop, 020-7307 2585.£9.99.