An investigation has been launched by medical regulators into the safety of metal hip replacements, leaving thousands of British patients fearful of the risk of being poisoned by their implants.

The Medicines and Healthcare products Regulatory Agency (MHRA) said it had taken "prompt action" over the safety concerns, but added the majority of people with the devices are at "low risk of developing any serious problems".

The action comes as an investigation by the Sunday Telegraph found more than 30,000 British patients have received the "metal-on-metal" (MoM) hip replacements which are feared to be more dangerous than previously thought.

Problems occur with such devices when friction between the metal ball and cup causes tiny metal filings to break off and potentially seep into the blood. These fragments can also cause a soft tissue reaction, destroying muscle and bone.

The newspaper said there are growing concerns that the implants could also cause "systemic toxicity" in the body, prompting the MHRA to start drawing up new advice for those fitted with them.

A spokesperson for the MHRA said: "On the evidence currently available, the majority of patients implanted with metal-on-metal hip replacements are at low risk of developing any serious problems.

"We are continuing to closely monitor all evidence. This needs more analysis before any conclusions can be drawn and further advice given.

"We have already taken prompt action to investigate safety concerns and have provided advice on patient management to relevant healthcare professionals."

In April 2010, the Government agency, which is responsible for ensuring that medicines and medical devices work and are acceptably safe, issued an alert to healthcare professionals over the safety of metal implants.

It came after some patients began suffering soft tissue reactions "to the wear debris associated with MoM articulations".

The MHRA advised that people fitted with the devices should undergo annual check-ups for five years following surgery.

It also said that those experiencing pain should be given tests to check the levels of cobalt and chromium in their blood, and an MRI or ultrasound scan to check for soft tissue reactions.

In September 2010, DePuy International Limited, a subsidiary of Johnson & Johnson, announced it was urgently recalling two types of its ASR metal hip replacement implants.

It came after data from the National Joint Registry of England And Wales found "failure rates" of 13% for the ASR XL Acetabular system and 12% for the ASR Hip Resurfacing System. However, a report by the British Hip Society seen by the Sunday Telegraph says failure rates of the Acetabular system could actually be as high as 50% six years after surgery.

Following the recall the MHRA advised orthopaedic surgeons to contact and monitor all patients given the implants.

More than 10,000 of the 40,000 Britons who have had metal hip replacements were fitted with the devices made by DePuy, according to the Sunday Telegraph.

Bozena Michalowska, of law firm Leigh Day & Co which is representing more than 300 victims of the MoM implants, said: "We have known for some time the perils of these implants and their widespread use has meant we are now seeing hundreds of cases from people who have had these implants.

"The MHRA needs to be fully able to assess these products before they are implanted and not just pick up the pieces many years after the damage has been done."