The slump in the value of City of London offices has been so severe that it is now the only real estate hub that offers a good deal for international investors.

No other office market has seen as swift an adjustment in price that would make sense for investors, according to the annual Money into Property report from consultancy DTZ, which covers 38 countries across Europe, Asia-Pacific and the Americas.

The report says the outlook for global property investment remains bleak, with no respite imminent from falling values.

For the UK, the period between 2008 and 2010 is forecast to see the worst cumulative returns for property since records began in 1921.

Globally, returns from office investments are forecast to drop by a fifth in 2009, with property prices stabilising only during 2010.

While values are stabilising in some centres, the softening economy will force rents down across all markets, according to DTZ.

In the UK, prime rents in the City and West End of London have fallen 31 per cent and 23 per cent since the peak in 2007. DTZ expects further falls of 14 per cent in the City and 20 per cent in the West End over the next two years.

But even if rents fall further, the prime City of London office market offers attractive returns.

DTZ's analysis examined various markets and predicts that London's West End, Madrid, Paris and Sydney will reach fair value at some point this year.

Most other markets will only be attractive investments in the second half of 2010. Frankfurt, New York, Shanghai and Tokyo will not get there until 2010.

For the first time since the series began in 1975, the value of invested stock declined, falling 10 per cent to £586bn ($936bn) in 2008.

Tony McGough, global head of forecasting at DTZ, said: "While deals are clearly continuing to happen across all markets, they currently tend to represent very selective opportunities, often driven by distressed seller situations or on the back of very high-quality tenants and long-term leases."

He said DTZ analysis indicated that markets "typically reach fair value, and are then able to represent a broader base of opportunities, before they reach the bottom", adding that the "hunting season" was not yet under way.