LONDON (Reuters) - Commercial property values sank by a bigger-than-expected 2 percent in June, extending the market's slide to more than 19 percent since it peaked a year ago, key data showed on Friday.

The valuation data from Investment Property Databank (IPD) showed retail properties led the way down, shedding 2.4 percent as the UK economy slowed and consumers felt the pinch from mounting job loss forecasts, falling house prices, and higher inflation.

The data, which led to a slight dip in property derivative spreads, also showed yields on UK commercial property rose to 5.69 percent in June from 5.53 percent in May.

Property yields measure rental income in proportion to building values. They move inversely to property prices and can be compared with borrowing cost benchmarks, such as five-year sterling interest rate swaps, which were trading at around 5.72 percent.

According to data from interbank broker Tullet Prebon, the bid price for December 2008 UK All-Property total returns swaps fell to minus 15 percent after the data, suggesting a total drop in commercial property capital values over the full course of the year of about 20 percent.

Total returns combine rental income and capital growth. The rental element is relatively stable at around 5 percent and so can be easily stripped out.

A spokesman for property services firm CB Richard Ellis, one of the biggest IPD contributors, said last week that British capital values fell by 1.3 percent in June.

The IPD data was based on a sample of 3,788 properties worth a combined 43.1 billion pounds and held by fund managers and property firms.

(Reporting by William Kemble-Diaz; Editing by David Cowell)

(See www.reutersrealestate.com for the global service for real estate professionals from Reuters).