Stunned vets could hardly believe their luck after an envelope containing £800 in cash landed on their doormat.

The package, stuffed with £20 notes, came with a message simply reading “a donation”, leaving staff at the Blue Cross animal hospital in Colliers Wood at a loss as to where the money came from.

But with funding for the hospital generated by public donations, the cash will be put to good use helping pets whose owners are unable to afford vet fees.

Tamsin Durston, Blue Cross head nurse at the centre in Merton High Street, said: “When we opened the post and discovered the wad of £20 notes we couldn’t believe our eyes.

“We are so used to seeing pet owners who struggle to get by that this kind of generosity is overwhelming.

“Our clients, and their animals, will join us in saying an enormous thank-you to the mystery animal lover who sent it to us.”

The charity treats an average of 500 sick and injured animals every week.