GWR Bristol is billed as a "contemporary and chart music and information station", but an Ofcom Content Sampling Report claims the station playing only 47% of contemporary music (music from within the last two years).
"Ofcom today issued a Yellow Card warning to GWR in Bristol for operating outside the music obligations of its format," the regulator said in a statement.
"If we find that these issues have been addressed when we monitor the station again, after Monday 23 March 2009, then the Yellow Card will be lifted. If GWR does not come back within format within the week we will move to sanctions, which could involve a fine or shortening of licence."
GWR owner Global Radio hit back, claiming, "GWR Bristol was asked to supply music logs to Ofcom by today, Friday 13 March. The requested information was supplied by yesterday, 12 March. Ofcom's decision was made without reference to either the station, or by talking to its owners, Global Radio.
"This is an extraordinary way for a regulator to behave by issuing a judgement before a deadline has passed. We do not accept this finding, and are considering our position."
More worrying for Global, however, was Ofcom's claim that GWR Bristol was not the only Global station to act outside its remit.
"We received similar complaints about a number of Global stations and we advise the licensee to take this finding into account for programming across all its 'contemporary hit radio' (CHR) stations," Ofcom says. "Ofcom has had conversations with the licensee about content on its CHR stations in the past."