GP use premium rateDoctors' surgeries in some of the UK's most deprived areas are using premium-rate numbers that can leave patients facing hefty bills, according to the Guardian newspaper.

The Welsh nationalist party, Plaid Cymru, has found that GPs' surgeries in the South Wales valleys are among those using such numbers. Calling the practice "against the spirit of the NHS", it claimed that such "stealth" charges would put pressure on some of the poorest people and could stop patients seeking treatment.

The Labour-controlled Welsh government said it was committed to stopping GPs using 0844 and 0845 numbers but said some surgeries were tied into contracts. Previously the administration in Cardiff has won praise for policies such as providing free prescriptions and phasing out charging for car parking in hospitals.

Plaid Cymru's shadow health minister at the Welsh assembly, Elin Jones, said: "It is hugely disappointing that GP surgeries across Wales are using expensive numbers for their patients to call. "We have all experienced the frustration of being on hold in an automated queue for what can seem like many minutes while trying to book an appointment. But to discover that such calls can be very expensive is concerning.

"Stealth charges, such as these, could deter people seeking treatment from their doctor, with the inevitable result that some people might end up in an accident and emergency unit instead. Regulations state that patients should not pay more to make calls to a GP practice than other calls in the area. I have written to the minister to clarify whether he considers the use of premium-rate numbers appropriate in our health service, and to stamp out this practice so that patients only pay local rate call charges."

Plaid said callers to the 0844 number from a mobile phone faced a charge of up to 44p per minute. It said some were also using 0845 numbers, which can be expensive from a mobile. A Welsh government spokesperson said there were only 18 practices using an 0844 or 0845 telephone number. Six of these offer a standard 01 number alongside to give patients the option of which number to use.

The spokesperson said: "The Welsh government is committed to ending the use of these numbers. It has not yet been possible to end their use entirely because practices are tied in to contracts with providers, but once contracts come to an end they should not be renewed. In the meantime, GP practices must consider reducing the costs of these calls to patients, by offering a call-back service."

It also said it was introducing a system across Wales called My Health Online, which gives patients the opportunity to contact their GP practice via the internet.

The issue of GPs using premium-rate phone numbers has also been controversial in England. In May the health secretary, Jeremy Hunt, criticised GP surgeries that subject patients to "long waits on the phone to get through, sometimes at premium rates".