Scientists have discovered why we have throbbing shoulders and lobster-red legs after too much exposure to the sun.

They have pinpointed the chemical that makes red skin smart and itch hours after the sun has gone in.

And the finding could also lead to the development of drugs to treat long-term, painful and hard-to-treat conditions such as arthritis.

In the study, the researchers from King's College London used UV lamps to burn tiny squares of skin on the arms of ten volunteers.

Two days later, when the reddened skin was at its most sensitive, they removed slivers and ran experiments to pin down why it was so painful.

They found several chemicals to be present in the burnt skin at higher levels than usual.

The main one was CXCL5. It attracts inflammatory white blood cells to the sunburnt skin, triggering pain and tenderness.

Further experiments showed that it was also present in high levels in sunburnt rats and when it was injected into those that had not been burnt, it made them highly sensitive to touch.

Professor Steve McMahon, one of the scientists involved and head of the London Pain Consortium, said the research might be relevant to other inflammatory conditions, such as arthritis.

I'm excited about where these findings could take us in terms of eventually developing a new type of analgesic for people who suffer from chronic pain, the Daily Mail quoted him as saying.