Half-Life 2 pits you against alien invaders

The game's publisher, Vivendi Universal, blamed the delay on the recent theft of the title's source code.

The news will disappoint fans of the original game, one of the most popular and widely acclaimed games of all time.

Since it first appeared five years ago, Half-Life has sold some eight million copies in its various versions.

Hacking effort

The sequel had originally been expected to be out by the end of September. It was then pencilled in for a release in time for Christmas.

But the theft of a key part of the game appears to have derailed those plans.

"Vivendi Universal Games is only the distributor of the product, which is produced by the US studio Valve and so it will up to them to make an announcement," said the head of Vivendi Universal Games, Christophe Ramboz.

Then and now: Facial detail is much better in Half-Life 2

"But it is true that the launch has been delayed by a theft of a part of the game," he told the Les Echos newspaper.

The makers of Half-Life 2, Valve, confirmed the theft of the game code last Friday after copies leaked onto the internet.

It said the leak followed a concerted hacking effort on the company's computers over a number of months.

The code has spread quickly online, appearing on websites and file-sharing tools such as Kazaa and BitTorrent.

Vivendi said it made up about a third of game. Most importantly, it includes the physics engine, which determines how the action in a game is shown.

The source code lies at the heart of a game and is kept a closely guarded secret to stop other people copying it.

The team at Valve has five years working on Half-Life 2, with an average of 30 people working on the title at any one time.

Valve have appealed to the millions of Half-Life fans to help them track down who leaked the code.

It has set up a specific e-mail account, [email protected], for people to send any information.