London (Reuters) - Online gaming firms faced their biggest-ever crisis on Monday after U.S. Congress passed legislation to end Internet gaming there, threatening jobs and wiping 3.5 billion pounds off share prices. Britain's PartyGaming, operator of leading Internet poker site PartyPoker.com, and rivals Sportingbet and 888 said they would likely pull out of the United States, their biggest source of revenue, and warned on future profits.