Sky-high rent to move Transport for London's offices to the proposed 66-storey "Shard of Glass" next to London Bridge station has drawn fierce criticism.

TfL could pay up to £13.5 million a year for 300 square feet in what is to become the tallest building in Western Europe.

London mayor Ken Livingstone claimed the company got an "impressively attractive" deal for the lower floors.

"We're not paying a premium for the wonderful view," he said yesterday. "They are looking down on the dustbins and a bus garage."

But he refused to reveal the bill to taxpayers.

During question time at the London Assembly, the Conservatives' Tony Arbour told the mayor: "If it is as good a deal as that, you should be trumpeting it from the roof tops.

"The problem is ... you keep all your property deals the most incredible secret."

The Shard, a 1,016ft tower of glass that will replace an existing 24-storey office block, is due for completion by 2010.

According to the Estates Gazette, the rent will be £45 per square foot - compared to £31.10 per square foot for the mayor's prime offices in City Hall.

Yet Mr Livingstone told the assembly: "I wouldn't work yourself up into a lather, because when you do know what we're paying for it, you would most probably give me a big kiss."

He said TfL only had a "verbal agreement" with the owner. "We're not in a position to trumpet how good a deal this is until we've actually got the owner of the Shard's contract"

The mayor wants to bring together TfL's scattered offices under one roof "within reach" of City Hall.

In 2000 local government spending regulations foiled his attempt to secure the new More London offices next to City Hall for TfL. The Shard was "as close as we're ever going to get", Mr Livingstone said.

However, Mr Arbour accused him of having a "conflict of interests", as he also gave planning permission for the building. Construction can only start once a certain amount of office space has been rented out.

Mr Livingstone replied that the Shard would help regenerate Southwark, "one of the most deprived and neglected areas of inner London".