THE value of Glasgow Prestwick airport has slumped dramatically after a downturn in passenger numbers ahead of a proposed sale by its New Zealand-based owner.

Financial figures published yesterday by Infratil put the value of Prestwick and Kent Manston airports at £33 million, down from £44m a year ago. Both airports were put on the market in March after Infratil chief executive Marco Bogoievski said they were not performing.

A buyer has yet to be identified for the Ayrshire airport, although Infratil yesterday announced the appointment of accountancy firm PwC to advise on the sale, which is expected to take place within the next 12 months. Passenger numbers have slumped to their lowest level in a decade at Prestwick following a decision by Ryanair to focus growth at Edinburgh airport.

Figures published earlier this month show 1.2 million people took flights in the 12 months to April 2012, around half the number recorded in 2007 following a rapid expansion of the low-cost aviation sector.

The financial statement published yesterday by Infratil said: "The key assumptions in these valuations include future passenger and freight volumes, commercial revenue yields, the ongoing operating and maintenance costs for each airport and the appropriate discount and capitalisation rates.

"The directors' valuation at 31 March, 2012, fair valued the airports at $64.7m (£32.7m) (2011: $92.7m."

Experts have contrasted the sale of Prestwick to that of Edinburgh, which attracted a number of interested parties after BAA put it on the market, and was eventually sold last month for £807m to Global Infrastructure Partners, which also owns Gatwick.

However, Cara Haffey, corporate finance director at PwC in Scotland and part of the Infratil advisory team, said Prestwick was a "critical asset".

She said: "Prestwick is a critical asset which offers potential investors the opportunity to invest and develop, attracting more airlines and routes, benefiting not only the region but the rest of the country."

Avaiation analysts have questioned whether Prestwick would be able to recover from the decline that began late in 2008. It has long been reliant almost entirely on services offered by Ryanair and has recently seen more competition.