The elongated glass pyramid, built atop a train station in a scruffy neighbourhood near the Thames, will open with 26 floors of vacant office space, and developers have to fill it at a time when rents are at the flattest in at least 50 years.

The Shard's developers are spared from market wrath solely because the building was built with funding by the deep-pocketed royal family of Qatar, rather than a publicly listed firm, said John Cahill, a property analyst at Investec.

"With the Shard's office floors still empty, it would be panic stations if it was a listed developer behind it," he said.

Long gone are the days in London's commercial real estate market when the Foster + Partners-designed 30 St Mary Axe, known as "the Gherkin" because of its oblong shape, opened in the financial district in April 2004 with all floors already let.

The Shard is just one of several skyscrapers now sprouting across London with nicknames that reflect the silhouettes they cast on the skyline. But with a financial crisis having blown in since architects first came up with designs for the "Walkie Talkie" and the "Cheese Grater", lettings have been muted.

Rents for the best offices in London's financial district - the yardstick used by Shard developer Irvine Sellar for the offices at the bottom of the tower - have been 55 pounds per square foot since September 2010, property consultant CBRE (CBG.N) said. That is the longest period of flat rents since its records began in 1960.

"The only reason rents haven't started to fall is the relatively low level of available space at the moment," said Kevin McCauley, head of central London research at CBRE, who expects rents to remain flat for the rest of the year.

The Walkie-Talkie, also known as 20 Fenchurch Street, is being developed by Land Securities (LAND.L) and Canary Wharf Group. The Leadenhall tower - the official name of the Cheese Grater - is being funded by British Land (BLND.L) and the property arm the Ontario, Canada city workers' retirement fund.

NEW MODEL

Developers say a wave of lease breaks and expiries over the next several years will prompt tenants to move into well-appointed new offices.

"Why would you drive around in a 1970s car when you can have a 2012 model?" said Sellar, an entrepreneur who began his career with a clothing store on London's Carnaby Street.

So far, it hasn't worked out that way. Work has halted at the Pinnacle skyscraper on Bishopsgate, which will remain a stump in the ground until a major slice is let. And developers of a neighbouring tower at 100 Bishopsgate say they will only begin once a large tenant is signed up.

If the climate is bothering the Qatari funders of the Shard, they did not say so at an opening event on Wednesday.

"Recovering our investment is a minor thing at the moment," said Sheikh Abdullah Bin Saoud Al Thani, governor of Qatar Central Bank.

The development cost of the Shard, a neighbouring office building called The Place and communal areas around London Bridge train station is about 1.5 billion pounds.

"We have confidence in the London market and a long relationship with London," he said, emphasising that a slowdown was part of a normal economic cycle.

The only tenant so far is the 195-room Shangri-La hotel, which will occupy floors 34 to 52 of the 87-storey tower. Sellar expects the rest of the building to be fully occupied by the end of 2014, conceding it was a long-term view he could take only because of the Qatari backing.

He will have to lure tenants like media and financial firms to venture to the opposite bank of the Thames from the City, the traditional financial district. He says the Shard's location will save commuters who arrive by train from walking across London bridge in the rain. Others are unconvinced.

"A lot of traditional City tenants refuse to cross the river to even have a look," said Simon Wainwright, managing director of property consultancy J Peiser Wainwright. "In a nutshell it's a bridge too far for many," he said.

Sellar scoffed. "That's absolutely ridiculous... We've been very selective as to who can even come and view the building. Just look up the road at Ernst & Young and PWC and you realise you are in the middle of a financial district."

Discussions are underway with tenants for about a third of the office space, he said.

The Qataris, who also own London's Harrods department store and the luxury One Hyde Park apartment scheme in Knightsbridge, are not the only rich foreigners buying into London real estate.

Overseas buyers have invested 15.8 billion pounds in London offices since 2010, 64 percent of the total, CBRE said. Middle Eastern investors accounted for 11 percent, or 2.8 billion pounds. And foreign ownership of the City financial district stands at 52 percent, Development Securities (DSC.L) said.

"Clearly the recent appreciation of Sterling has had little effect on overseas investor's views that central London is providing both good value and a safe haven," said Simon Barrowcliff, a CBRE executive director.

But the next wave of investors do not appear to be putting so much money into skyscrapers. Ken Shuttleworth, the architect who led the team behind the Gherkin while at Foster + Partners, said no plans for London skyscrapers were crossing his desk.

"In the current economic climate, we are basically only working on skyscrapers in the Far East," he said. (Reporting by Tom Bill)

• A Permanent Economic Emergency