Tourists craning their necks to admire the artwork by Michelangelo in the Sistine Chapel are being targeted by thieves taking advantage of the crowds.

Holidaymakers have been leaving warnings on travel websites urging others not to get stung when they visit Vatican City.

It has prompted calls for the Vatican to improve security and restrict the number of visitors it lets in during the day.

Official guides have even threatened to go on strike if something is not done. A similar protest recently took place at the Louvre, in Paris.

Angela Bolton, a guide from Kent, told The Times: ‘The situation is getting out of hand.

‘The Vatican lets in as many people as they can. Up to 30,000 people have been going through the museum on a single day, which means crowds like those on the Tokyo Underground.’

The Sistine Chapel, or  Cappella Sistina in Italian, takes its name from the man who commissioned it, Pope Sixtus IV.

Some 25,000 people a day, or five million people a year, visit the chapel.

Entry to the Vatican Museums and Sistine Chapel costs €16, earning the Vatican an annual revenue of around €80 million or £70 million a year.

The Sistine Chapel is most famous for Michelangelo’s frescoes, but long before Michelangelo, Sisto commissioned painters such as Botticelli to fresco the two long walls of the chapel.