Online shopping sites are costing the city tens of millions of dollars a year in lost revenue by not paying state and city sales taxes, and the figure is likely to increase in coming years, according to a report by the Independent Budget Office released yesterday.

While popular sites that also have physical stores in the city, such as CompUSA or Barnes and Noble, have to collect sale taxes on Internet purchases, purely online companies such as Amazon.com are not required to do so. This leaves the responsibility for paying the sales tax with consumers, who rarely report their online purchases.

According to the IBO, sales on these Web sites cost New York more than $50 million in uncollected local and state tax revenue between June 2006 and July 2007 — $29 million of which failed to reach New York City’s coffers. The chief of staff for the IBO, Doug Turetsky, said yesterday in an interview that because online sales are growing, the lost revenue is likely to increase in coming years if no action is taken to force sites to collect the taxes.

A state law passed in June, nicknamed the “Amazon tax,” could net some of the lost revenue by forcing certain Web sites that are not based in New York but have affiliates or offices in the state to collect taxes on online purchases, the report says. The state projects that it will collect nearly $50 million and the city $27 million this year under the new law, and nearly $100 million a year in state revenue and $60 million a year in city revenue by 2012. The law may not go into effect, however, as Amazon.com and another shopping site, Overstock.com, are mounting separate legal challenges arguing that it is unconstitutional.