Doctors have voiced concern about the growing use of foreign GPs to cover weekend shifts.
Many UK doctors opted out of providing weekend care last year
GPs from across Europe are flying into the UK to work on Saturdays and Sundays after nine in 10 UK family doctors opted out of the work last year.
But leading UK medics said patient care was being compromised because the GPs were unfamiliar with the NHS system.
They claimed it had led to incorrect prescriptions being issued and unnecessary referrals to hospital.
There are no figures for how many foreign GPs are being brought in to cover the out-of-hours service.
But General Medical Council statistics show that the number of foreign doctors from many European countries registering to work in the UK rose sharply last year - when UK GPs gave up the weekend cover.
The total number of German doctors (not just GPs) registering shot up from 383 in 2003 to 771 in 2004.
Medics from France also increased by 41 to 150, while those from Italy rose from 245 to 308.
Health bosses in the North East and north Wales are known to use German doctors.
Any GP wanting to practice in the UK - even for only weekend work - needs to register with the GMC - although doctors from the European Economic Area are automatically allowed on.
But Dr Peter Swinyard, a member of the British Medical Association's GPs committee, said it was a worrying trend.
"These doctors are not familiar with the NHS and what we see is a tendency to refer on to secondary care (hospitals) and (bad) prescribing which we have to sort out on a Monday.
"This is very different from foreign doctors who come to work full time in the UK as they become familiar with the system."
Dr Swinyard added by the time travel and accommodation expenses were paid, the cost of European GPs was double what it was for a UK doctor.
"If they offered this to UK doctors you might get more doing it."
Dr Surendra Kumar, president of the British International Doctors Association and a GP, also raised concerns about care.
"I have heard of doctors trying to charge for services which they shouldn't and inappropriate use of antibiotics.
"It is concerning and is simply down to the doctors not knowing about health care in this country."
A spokeswoman for the GMC said all doctors who were registered to work in the UK had the full medical qualifications.
And Dr Michael Dixon, who is a GP and also chairman of the NHS Alliance, which represents primary care trusts, said: "I don't think we can have it both ways.
"We can't say we don't want to do out-of-hours and then start complaining when someone else is doing it."