Giant panda Tian Tian artificially inseminated at Edinburgh Zoo

The UK's only female giant panda Tian Tian has been artificially inseminated for a fourth time at Edinburgh Zoo.

Experts carried out the procedure on Sunday evening after tests showed she had reached peak fertility.

Tian Tian, which means Sweetie, has failed to produce a cub despite repeated artificial inseminations since her arrival at the zoo in 2011.

The zoo is open but the panda enclosure will remain closed to visitors until 5 May.

Panda reproduction is notoriously difficult, partly due to the very short breeding window with ovulation occurring only once a year.

Staff had hoped Tian Tian would mate naturally with the zoo's male giant panda Yang Guang but when this did not happen, a decision was taken to artificially inseminate her using Yang Guang's sperm.

Colleagues from the China Conservation and Research Centre for the Giant Panda (CCRCGP), the Leibnitz Institute for Zoo and Wildlife Research (IZW) and Roslin Embryology assisted the zoo's staff during the procedure.

Iain Valentine, director of giant pandas for The Royal Zoological Society of Scotland, said it would be some time before they knew if Tian Tian had conceived.

Writing in his blog, he said: "We continue to believe that it's important biologically for Tian Tian, a female in her prime, to breed and reproduce and add to a vital ex-situ population outside of China.

"If we can successfully assist Tian Tian to carry to full term, we have no doubt that she'll be an excellent mother and both our male and female's genetics will be preserved in future giant panda generations."

Some animal rights campaigners have in the past criticised the use of artificial insemination and attempts to breed the animals in captivity as there is no intention to return to them to the wild.

The two giant pandas arrived in Scotland in December 2011 and are being rented by Edinburgh Zoo from the Chinese government for a decade for an annual fee of about £600,000.

Tian Tian had previously given birth to twins in China but all attempts to produce a cub at Edinburgh Zoo have failed.

Zoo staff believe she may have been pregnant on a number of occasions but pandas sometimes re-absorb the foetus during the course of the pregnancy.