The Sherlock star wants Cameron to follow through on his promise to introduce legislation to prohibit wild animals from performing in the travelling shows.

In the letter, sent on behalf of People for the Ethical Treatment of Animals (PETA), the father-of-two writes, "As a father, I'd like to see my children grow up in a country where animals are treated with respect, not as objects of ridicule... Allowing circuses with wild animals to continue to operate in the country sends the message to children that it's OK to dominate animals and ignore any pain and suffering that they're experiencing - which, of course, it's not."

Actor Michael Sheen and girl band Little Mix have also lent their support to the cause.