The government has been warned it must do more to prepare the NHS for changes in GPs' working hours next year.

In a new report released on Friday the Commons health select committee said the opt out from providing out-of-hours services which comes into effect on December 31 has not been fully thought through.

The MPs found that primary care trusts are "in varying states of readiness" for the change which will allow family doctors not to work nights or weekends. Around nine out of 10 GPs are expected to take up the opportunity.

The cross-party committee also warned that more funding may be required to cover the gap in the provision of services that will open as the changes come into affect during 2005.

And the report recommended that ministers insist on minimum standards of care to be provided by the various parties which will take over the work, including nurses, paramedics and NHS Direct.

Committee chairman David Hinchliffe said out-of-hours services are an "integral part of the NHS".

"The scale of the service they provide is considerable - nine million people receive GP out-of-hours care every year, and the 'out-of-hours' period in fact accounts for two-thirds of each week," he said.

"GP out-of-hours services also play a crucial role in controlling demand on other parts of the NHS, such as A and E and ambulance services - one group of GPs told us that without an effective out-of-hours service, emergency services in their area could have to cope with an additional 25,000 patients every month.

"Up until now it has been the responsibility of individual GPs to provide out-of-hours care for their patients. But from 31 December 2004, GPs will be able to opt out of this cover, meaning responsibility for out-of-hours services will fall to their local primary care trusts.

"The BMA told us that as many as 90 per cent of GPs are likely to opt out. This will mean a large scale change across the country in the way out-of-hours services are managed, funded and in some cases, delivered."