Thousands of gamers were unable to play the multiplayer test version of Halo 3 on Wednesday due to technical problems.
Players were unable to download the beta program and hundreds of people left messages on the maker's official website complaining about the issue.
Bungie has now fixed the problem and extended the three-week beta period by four days.
The problems will be embarrassing to Microsoft, which is touting Halo 3 as one of the year's biggest games.
"We realise this has been a pain for many and an inconvenience at the very least," wrote Bungie's Community Manager Brian Jarrard on the firm's website.
He added: "Given the circumstances, we've decided to extend the duration of the Halo 3 Beta from 6 June to 10 June.
'Full day'
"So, you lost almost a full day of play today but to make up for it we're tacking on four more days (and a weekend at that)."
Fans were offered the chance to take part in the beta by downloading the code included in a separate game, Crackdown.
The downloads were supposed to start at 1300 BST on Wednesday but gamers found the option in the game remained locked.
Microsoft has not explained what caused the delay but did say the problem was not due to Crackdown or the beta itself.
Halo 3 will be released on 25 September in the US, and a day later in the UK.
Halo 2 earned $125m in sales in its first 24 hours of availability when it was released in 2004.
The first two Halo games have sold a combined 11 million units, according to Microsoft.
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