Ministers face opposition calls to change the GP out-of-hours cover ­system as the case of a foreign doctor who accidentally killed a patient on his first UK shift highlighted cracks in the way the service is operated.

The Conservatives said the tragedy, revealed by the Guardian on Monday, was a wake-up call, and the Liberal ­ Democrats said relying on doctors being flown in for weekend shifts was unsustainable.

The political pressure over the case of Daniel Ubani, a German national of ­Nigerian origin, came as the family of David Gray, who died from a lethal dose of a painkiller administered by the ­doctor in February last year, accused German authorities of potentially putting more lives at risk because of the slow process involved in investigating his fitness to practise.

The 24/7 cover arrangements for GP services, which 9 million people in ­England use, result from new contracts arranged with family doctors in 2004.

The changes, which meant many ­doctors felt able to withdraw from out-of-hours work, left organisation of services to local ­primary care trusts (PCTs).

These commission local doctor co-ops or private companies, hiring a mix of local GPs, locums and foreign doctors to provide care. More than 100 foreign doctors were used in just 10% of PCTs over a period of 12 months, figures suggested last year.

Critics say alternative out-of-hours ­provision often confuses patients. ­Services include drop-in centres and ­telephone advice lines, and patients often prefer to use local casualty departments, the critics say.

Andrew Lansley, the ­Conservative health ­spokesman, said: "It is deeply worrying that many primary care trusts have cut corners on the quality of care patients receive in order to tick boxes to hit ­government targets."

A Tory government would make local GPs responsible for out-of-hours care again, he said. "They are the ones who know their patients best and know how to deliver the quality care that they [the patients] need."

The Liberal Democrats called for an independent review of the whole system to address the problems Gray's death ­­­­had highlighted. "A trusted ­out-of-hours ­service is vital for patients. The ­current ­system is not working and ­placing ­enormous pressure on hospital A&E departments," said their health spokesman, Norman Lamb. "

David Gray's son Rory works in Germany. He described the apparent slowness of medical authorities there in instigating professional proceedings against Ubani as "incredible".

"It is totally irresponsible," he added. "People will die if he practises like he did in the UK."

The NHS is to investigate Take Care Now, which held the contract for the area of Cambridgeshire in which Gray died.