Wales' Labour government pledged in its 2011 manifesto that it would deliver ‘improved access to GP surgeries in the evenings and Saturday mornings’.
But GPC Wales chairman Dr David Bailey said Welsh GP practices could not be forced to open at the weekends.
‘We’ve told the Welsh government that we’re not prepared to negotiate on a separate set of core hours for GPs in Wales.’
Dr Bailey said that under the GMS contract, GP practices are required to work Monday to Friday between 8am and 6.30pm.
Any change to those hours would require a contract negotiation, something which the GPC is not prepared to consider, he said.
Some practices may choose to open for additional hours under the extended hours DES, Dr Bailey added.
Commissioning more out-of-hours care may also be an option for providing extra accessibility, he said.
The manifesto also revealed Labour's commitment to instigating annual GP-led health checks for everyone over 50 years old.
Dr Bailey said that providing a health check for everyone over the age of 50 was unnecessary.
‘What we don’t want is hundreds of thousands of appointments being taken up by people who don’t need to see their GP.’
Dr Bailey said that the GPC was having discussions with the Welsh government about ways to reduce the number of people being offered the health check.
A Welsh Government spokesperson said: 'We are looking in the first instance to GPs to work more flexibly within their contracted hours, to provide appointments convenient to their patients. However, where there is demand beyond normal working hours, we would encourage GPs to take-up the provision of extended surgery hours.
'We are also committed to developing a more preventative approach to healthcare and by introducing a programme of annual health checks for people over the age of 50, will enable clinicians to intervene early and, ultimately, save lives.'