Modern office buildings are all the rage in central London, with a swath of companies including UBS Group AG moving into new premises in the past 12 months. The problem for landlords is finding someone to replace them.

A growing number of unoccupied older properties caused the overall office vacancy rate to climb to 5.8 per cent at the end of the first quarter from 3.9 per cent a year earlier, according to data compiled by Deloitte LLP. That was the biggest increase since 2009.

"The demand for new space is still there; the demand for second-hand space isn't necessarily as hot," says Shaun Dawson, a research manager at the firm.

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Investors pulled back from London real estate immediately after last year's Brexit vote, fearing that Britain's departure from the European Union would cause a collapse in demand for office space. While high-profile lease agreements by companies including Apple Inc. and Deutsche Bank AG have helped ease concerns since the June 23 vote, the latest data from Deloitte show there are limits to the market's recovery.

The amount of office space available to rent climbed 36 per cent in 2016 and by a further 19 per cent in the first quarter, Deloitte says in the report. That's almost entirely made up of space that has become available as a result of companies downsizing or moving offices, the data show.

UBS, for example, completed a large office move last year, vacating several older buildings in favour of a single new London headquarters.