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It is spending £40m on a new warehouse which will have an initial annual sales capacity of £1bn by 2012/13.
Sales increased 58% in the nine weeks to June 2010 and operating profit increased 46% between 2008/09 and 2009/10. The company did not pay a dividend, choosing to reinvest the capital.
IT expenditure increased £700,000 as Asos focused on developing and enhancing core systems, said the company.
During the year it replaced its warehouse management system and completed its migration to a tier-one datacentre.
It also started replacing its buying and merchandising system, which will be completed in 2011.
The coming year will see the launch of the Asos Marketplace which will allow customers to re-sell unwanted clothes. There are also plans to launch a US site in September 2010, and French and German websites later this year. Asos says its US market is growing at a rate of 180% a year.
CEO Nick Robertson said: "We believe online fashion will continue to outperform traditional retail channels. We are at the leading edge of our sector and see enormous potential to drive our business forward, both in the UK and internationally."