Medical regulators have launched an investigation into the safety of metal hip replacements amid fears that thousands of British patients are at risk of being poisoned by the 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 spokesman 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.