In a statement detailing the banning of the accounts Valve said this system helped identify who had got hold of pirated copies.

"The method used was extremely easy for Valve to trace and confirm, and so there is no question that the accounts disabled were used to try and illegally obtain Half-Life 2," read the statement.

Valve spokesman Doug Lombardi said that the company had not yet released sales figures for the game and would not say what proportion of all Steam players the 20,000 represented.

One effect of Steam's popularity has been to limit the copies of the game sold in shops and artificially depress the game's ranking in the top ten.

Even so the title debuted at No 3 in the UK charts.

Valve also said that a small number of accounts were closed because people were using stolen credit cards to buy copies of the game or were using stolen Steam accounts.