By Keith Weir LONDON (Reuters) – Bookmakers are preparing for a tax avoidance crackdown on online gambling in Britain that will cost the industry 300 million pounds ($490 million) a year, putting a brake on the fastest growing part of their businesses. Many bookmakers have set up internet operations in territories such as Gibraltar, allowing them to sign up British gamblers while benefiting from benign local tax regimes. Britains government, however, is taking a tough line on tax avoidance as it seeks to swell the Treasurys coffers, planning a 15 percent duty on bookmakers online winnings from British-based customers from next December. That would bring the tax regime into line with the duties bookmakers pay on takings in betting shops and provide a rare example of taxation policy catching up with the internet age.