Medical care at nights and weekends when GP surgeries are closed is a shambles with slow response times and escalating costs, according to a report by the National Audit Office.

Family doctors are paid up to £4,000 for providing medical cover over a single weekend but most patients face long waits, says the government watchdog. Foreign doctors are cashing in on out-of-hours pay by flying in to provide cover for a few shifts over a weekend or bank holiday before flying home again. The generous payments are also boosting the income of British GPs who were reported last month to be earning up to £250,000 a year.

From January last year, responsibility for most out- of-hours cover was switched from GPs to primary care trusts, who frequently contract out the work to private companies. But the costs of the new service have risen by more than £200m, increasing the financial pressure on NHS trusts, and it is failing to meet its quality standards, the NAO said.

Instead of visiting patients in their own homes, as in the past, most services now require patients to attend a medical centre at night or weekends with one in 10 receiving a home visit.

A survey of patients conducted for the NAO found four out of 10 said they waited at least two hours to see a doctor and one in 10 waited more than four hours.

Chris Shapcott, of the NAO, said: "Performance was decidedly patchy. On response times, very few could report good results."

Once patients got to see a doctor or nurse, most were satisfied with the care they received, but one in five said it was poor. In the past GPs did their own night and weekend calls or hired deputising services to cover for them. But providing the cover became increasingly unpopular and the health department offered GPs the choice of opting out in return for giving up £6,000 of their annual income.

Since January 2005, the local PCT, or private company, running the out-of- hours service has hired doctors to work shifts at up to £141 an hour, the NAO found. For two 15-hour shifts from 8am to 11pm on Saturday and Sunday, a doctor could earn up to £4,230. The number of doctors from Germany, France and Italy registering to practice in the UK has doubled, according to the General Medical Council.

The NAO said the health department saved £180m from the £6,000 sliced off GPs' incomes but the total cost of the out-of-hours service in 2004-05 was £392m - £212m more.

The department allocated £312m, £70m short of the £392m required. "That created a problem for the PCTs," Mr Shapcott said.

Lord Warner, the Health minister, said all PCTs would be told to improve their cost effectiveness and performance. "The NAO report confirms the NHS is on the right track towards providing quality, round-the-clock services with high levels of patient satisfaction. Evening and weekend services ... are greatly improved." Andrew Lansley, the shadow Health Secretary, said: "We are paying much more money for a worse service."