The British company developing the European Union's tallest building defended their security measures overnight after it emerged that a group of "urban explorers" had broken into the skyscraper.

Spectacular photographs apparently showing trespassers leaning over scaffolding at the top of London's 310-metre Shard tower as the city lights twinkled below appeared online over the weekend on the Place Hacking blog.

Sellar Property Group, the developer scheduled to finish work on the enormous glass-clad building next month, admitted there had been a break-in but said it had stepped up security in response.

"We believe this is an incident which took place around December 2010," a Sellar spokesman said.

"The breach was discovered very soon afterwards and security immediately tightened.

"Today security on the site is tight with 14 night-time security guards on duty continuously who cover all areas, as well as 25 CCTV cameras in operation together with a ground floor level laser alarm system."

But Bradley L Garrett, an American graduate who posted some of the photographs on his blog, said he and a group of fellow "urban explorers" had climbed the Shard's stairs "half a dozen" times.

"It is impossible to secure a site that big," the 31-year-old said.

"The security guards that work on the site are only human.

"No one wants to sit there watching cameras 12 hours a day," added the Californian, who completed a PhD on urban exploration at the University of London in February.

The group first climbed the Shard to watch the New Year's Eve fireworks over London in December 2010, Mr Garrett added.

The Sellar Property Group spokesman said he could not comment on Mr Garrett's claim that climbers broke into the Shard more than once, "as there is no further photographic evidence to support this claim".

Another blog, SilentUK, said a "large contingent" of climbers had managed to break into the construction site and walk up the Shard's 95 floors.

"The wind howled, the crane upon which we stood creaked and swayed, London stretching as far as the eye could see," the group said in a post alongside more panoramic shots of London, apparently taken at the tower's summit.

Other photos posted on the site show climbers scaling the Brooklyn Bridge in New York and the Palace of Justice in Brussels.

AFP