Two UK patients have had their hip implants replaced after a packaging error meant they were given the wrong size, it has emerged.
There are more than 60 different makes of implants the UK
Around half of the 185 implants involved, which have since been recalled by manufacturer Smith & Nephew, had been distributed in the UK.
It is not clear how many more patients may have received one of these.
The company said hospitals were being instructed not to fit any more and to monitor any patients at risk.
The Medicines and Healthcare products Regulatory Agency has issued an alert to UK professionals about the Birmingham Hip Resurfacing implants.
The MHRA said about 83 affected devices may have been in circulation in the UK, but some have since been returned unused.
A spokesman urged patients not to be alarmed by the news.
"We do not want people to panic. The vast majority of people who have received a hip replacement will not be affected.
"But, clearly, anyone in doubt should contact their surgeon."
The manufacturer recalled some affected products in March 2007 and then extended the recall in June and again in July, he added.
Heath professionals have been advised to review the post operative X-rays of patients already implanted with affected devices to identify any mismatch of head and cup size.
A mismatch may not cause the patient any immediate symptoms or problems, but could with time.
A spokesman for Smith & Nephew said some patients were also having their blood monitored for any unusual wear that could be happening from a possible mismatch.
"Where patients may have been given these hips they are already being monitored."
He said the implants were incorrectly packaged by a subcontractor and, as a result, different sizes of acetabular cup were mixed together.
Some 50,000 patients have been fitted with Birmingham Hip Resurfacing implants over recent years.