By Boris Groendahl

VIENNA (Reuters) - Austrian brickmaker Wienerberger (WBSV.VI: Quote, Profile, Research) warned on Thursday that earnings would fall this year as residential construction in Britain "collapsed" and U.S. building fell sharper than expected, sending its shares down as much as 23 percent.

Wienerberger, the world's largest brickmaker, said in a statement business started to worsen in the second quarter, leading to a decline in quarterly revenues and a 10 percent drop in core earnings for the first six months of the year.

Its shares were down 17 percent at 18.95 euros by 10:30 a.m. British time after hitting the lowest level in almost five years, leading a 2.8 percent drop of the DJ European Construction and Materials index as fears of further profit warnings weighed.

"This came with a ferocity that I didn't expect," said UniCredit analyst Peter Bauernfried. "I would have thought that they would still manage to raise earnings."

Wienerberger had earnings before interest, tax, depreciation and amortisation (EBITDA) of 551.2 million euros ($866.5 million) in 2007. It said in May it expected EBITDA to grow by more than the industry average this year.

Analysts polled by Reuters Estimates on average forecast a 6 percent EBITDA rise to 586.7 million euros this year.

Credit Suisse warned just before Wienerberger's statement it expected more building materials makers to cut forecasts. It named Saint-Gobain (SGOB.PA: Quote, Profile, Research), Italcementi (ITAI.MI: Quote, Profile, Research) and Holcim (HOLN.VX: Quote, Profile, Research) as the most likely to warn on profits.

Also before Wienerberger's statement, Irish bank Davy cut its estimates for the Austrian company, predicting an EBITDA drop. "In the short term, the market will be shocked by the scale of downgrades," Davy said in a note to clients.