Tony Blair delivered an ultimatum to doctors yesterday to change the way they worked or risk losing extra funds pledged to the health service.

The Prime Minister told a conference of the British Medical Association, in Harrogate, that the Government would be unable to protect the NHS unless doctors agreed to modernise the service in line with public expectations.

Mr Blair, speaking via a video link from Downing Street, warned that public support for the NHS would wane if it did not change, which could give the upper hand to those seeking its privatisation.

"I can get the extra resources into the NHS - I can do that," he said."We are prepared to honour our part of the bargain but I do ask you to work with us so that we can get the change that we need."

He said patients wanted easier access to treatment and advice, more care provided in GP surgeries and a seamless service between hospital and home. "The only way to get support from the public for extra resources in the NHS is if it is accompanied by change that brings better services for patients," he said.

He was challenged during a question-and-answer session by GPs who claimed they were working under intolerable pressure with inadequate resources. Mary Church, a GP in Glasgow, said she was working 70 hours a week and was "drowning in a sea of paperwork". The Government expected surgeries to open 24 hours a day, she said. "When do you expect us to sleep?"

Mr Blair said the Government was recruiting more staffand investing in measures such as NHS Direct, the telephone helpline, to take the pressure off GPs, but he said: "I do ask you to work with us because my fear is that if you don't put in these changes then people will get increasingly demoralised about whether the health service can get back on his feet."

In a barbed reference to critics who have called for extra private investment to save the NHS, he added: "If we don't modernise, then the agenda will move to those who don't want to rebuild the NHS at all."

Mr Blair, the first prime minister to address a medical conference by video link, seemed unaware of the limitations of the technology. He drew laughter from the audience of 500 when he beckoned to an aide off screen to refill his coffee cup.

Answering criticism of the "instant access" culture of NHS Direct and walk-in medical centres, Mr Blair cited research by Sheffield University showing 97 per cent of patients had found NHS Direct valuable. "People want faster and more convenient access and we have got to build on that."

The demand that doctors agree a deal with the Government - modernisation in return for extra cash - was driven home by the Secretary of State for Health, Alan Milburn, who called for "tailor-made" services for the 21st century NHS to replace the "mass-produced services" of the 20th century

"Investment plus reform is the key. Throwing more money into it [the NHS] without transforming the services it delivers would break public confidence for good," he said.

But doctors at the conferenceaccused Mr Milburn of failing to face reality. Dr Jonathan Reggler, a GP in Marlow, Buckinghamshire, said: "When will you ... tell the public that the NHS cannot do everything. All I've heard today is fantasy given your funding."

Dr John Chisholm, chairman of the BMA GP's committee, said doctors wereperplexed about how the reforms could be achieved given the limited resources. He said the Prime Minister had left no doubt that the extra cash promised might be tied to specific modernisation projects such as NHS Direct. "There was an element of 'change is happening and you had better bloody well like it'," he said.