In its European office report for investors in commercial mortgage-backed securities, Moody's also found that Dublin and Warsaw are the cities where rents are most likely to drop because more space is being built than companies are taking up.

Conditions may worsen because the global tightening of credit resulting from the subprime mortgage crisis could curb demand from investment banks for office space, according to the ratings company.

The City of London, as the UK capital's main business district is called, "is likely to be most at risk from a downturn in letting activity due to its high proportion of financialservices occupiers".

Canary Wharf Group had a vacancy rate of 0.5% out of 7.9 million square feet (734,000sq m), equivalent to less than 40,000sq ft, in its completed buildings at the end of June.

Another 1.3 million feet of space that's being built has already been either sold or leased. The company is also proceeding with a building of 400,000sq ft for which it has yet to find a tenant.

The City of London had 4.9 million square feet of space under construction in the first quarter, of which only 400,000sq ft had been leased at the time, according to the latest survey by realtors Drivers Jonas.

Frankfurt could also be affected by any drop in demand from banks, Moody's said. Still, German office markets as a whole are improving as vacancies decline, according to the company.

Moody's report covers 24 cities and scores them out of 100, with higher scores more attractive to investors in commercial mortgage-backed securities. London's Docklands was given a rating of 86, compared with Dublin's score of zero. The Irish capital has been one of Europe's worst markets on the methodology used by Moody's since the company started its reports in 2000.

The overall rating for all markets was 63, unchanged from a year earlier.

The Scottish cities of Glasgow and Edinburgh had the best ratings after London's Docklands, followed by Madrid, the La Defense district of Paris and London's West End, which charges the highest rents in the world.

Other cities covered by the report include Amsterdam, Barcelona, Brussels, Milan, Munich, Rotterdam and Stockholm.