The new contract was introduced in 2004

The Lib Dem data from 92 of the 152 primary care trusts show problems in areas such as assessing patients.

The Lib Dems said the figures were disturbing, but the government said the NHS must improve care where necessary.

PCTs took on responsibility for weekend and night-time care after 90% of GPs opted out under a new contract in 2004.

The changes were introduced in a bid to ensure the GPs were fresh for weekday work and have led PCTs to employ private firms or groups of independent doctors and other health staff to provide cover.

These very disturbing statistics show that many primary care trusts are failing to meet standards of care which the government itself says is crucial Norman Lamb, Lib Dem health spokesman

Critics have always maintained that the service has got worse with patients struggling to get through to doctors out-of-hours, while GP pay has broken through the �100,000 barrier.

Last month the leading doctor insurance firms said serious complaints regarding out-of-hours care had soared since the new contract came in.

The latest data adds weight to the argument of critics, showing PCTs are struggling to ensure their providers keep to official standards.

Nearly half of trusts do not meet standards in answering patient call quickly, while a third do not the telephone clinical assessment guidelines, which are key to identifying life-threatening conditions.

Just over a quarter do not meet standards in starting face-to-face consultations on time, while 23% struggle on sending details of out-of-hours consultations to day-time GPs by 8am the next working day.

'Disturbing'

Lib Dem health spokesman Norman Lamb said: "These very disturbing statistics show that many primary care trusts are failing to meet standards of care which the government itself says is crucial.

"Out-of-hours care looks at potentially life and death situations. Answering calls quickly and providing a swift and accurate phone diagnosis can save lives."

And a Patients Association spokeswoman added: "There are some serious concerns about out-of-hours care and this seems to show they are well founded."

A Department of Health spokesman said previous research has shown patients were generally happy with the out-of-hours care they received.

But he added: "Where a provider is failing to meet the quality requirements, PCTs as commissioners of the service, and strategic health authorities must act to support out-of-hours providers to improve their performance."

The Lib Dems released the figures as they unveiled their health plans, which include local health taxes, elected boards to govern services and a patient contract to guarantee good care.