Scientists have discovered that cod liver oil can repair arthritic joints as well as lubricating healthy ones.

They say Granny really did know best when she insisted on a daily dose, because it halts the inflammation and destruction of joint cartilage caused by arthritis.

And they believe supplements could help thousands of patients waiting in agony for hip and knee replacements by beginning a reversal of the degenerative process.

More than two million people in Britain have been diagnosed with osteoarthritis.

The researchers, at Cardiff University, say there are scientific reasons for the long-held belief that cod liver oil is good for joint health. Professor Bruce Caterson, who presented his findings in London yesterday, said it was never too late to start repairing the damage caused by arthritis.

'Cod liver oil can be very helpful and beneficial in arthritic diseases,' he said.

'We've been able to show that we can slow down or remove the activities of the enzymes that degrade cartilage in arthritis and also keep inflammation down to a lower level.'

The latest research shows the importance to joint health of essential fatty acids - called Omega-3 fatty acids - contained in cod liver oil and other fish oils.

These are incorporated by the body into the cartilage cells damaged in arthritis sufferers.

Cartilage is the protective material that covers bones around the joints to stop them grating.

The scientists tested the effect of Omega-3 fatty acids on the cartilage from the discarded arthritic knees of patients who had had knee replacements.

Some samples were treated with the substance for 24 hours, while others were not.

A chemical was added to mimic an inflammatory response.

When researchers looked at the untreated cartilage they found enzymes responsible for destroying it.

But in the samples treated with Omega-3 fatty acids the enzymes had been 'switched off'.

Fergus Logan, chief executive of the Arthritis Research Campaign, which partly funded the research, said: 'These findings are extremely exciting and offer a scientific basis for why cod liver oil helps people with arthritis - and has done for many years.'