Great Portland Estates suffered a 15.3% plunge in its net asset value in six months, as the worsening financial markets took their toll on its London portfolio.

The central London office specialist said this morning that its NAV had dropped to 493p a share in the six months to 30 September, which, though severe, was not as bad as the 20.7% fall experienced by Land Securities in the same period.

The drop in NAV was driven by a 9.5% decline in the value of the portfolio to £1.42bn.

Wholly-owned properties were valued at £1bn and properties owned in four 50:50 joint ventures were valued in total at £834m.

The second quarter's valuation decline of 5.9% was greater than that experienced in the first quarter of 4% due to the worsening of market conditions during September.

‘For as long as the exceptional market environment continues we anticipate that the portfolio valuation will come under further negative pressure,’ chief executive Toby Courtauld warned.

The drivers of the valuation fall were: a 54 basis point rise in the equivalent yield to 6.09%; a reduction of 2.1% in rental values; and a fall of 7.2% in the value of development properties.

Courtauld said Great Portland was ‘well placed’ to sail through the economic storms. ‘We have been planning for a downturn for more than 12 months; we have been net sellers of property; we have finished and let developments without starting new ones.

‘Today, our priorities are capital conservation and cash flow, maximising occupancy levels and crystallising reversions.’