Amazon.co.uk has delivered copies of Grand Theft Auto 5 ahead of the official launch on Tuesday, breaching a strict embargo set by the game's maker.
The many customers who had pre-ordered the title began receiving it on Saturday, and some have been quick to announce the fact via social media.
Rockstar North, the title's Scottish creator, is reportedly investigating but has yet to comment publicly.
Will Guyatt, spokesman for video and entertainment site IGN.com, told the BBC news website: "I got an early copy on Saturday, which I paid for myself, and was delighted on a personal level. But it's obviously going to cause problems for Rockstar.
"There are about 2,000 shops stocking the game ready for launch on Tuesday, so the number of people posting spoilers online is a bit sad", he said.
Grand Theft Auto 5 may turn out to be the most expensive video game ever made.
Guyatt said he had ordered his copy of the 18-rated violent crime game in March, but others in his office who had ordered it at a later date had not yet received their copies.
"This kind of thing has always happened in the games industry, but with the rise of social media a lot more people know about it now", Guyatt said.
"I can't see what Rockstar can do about it."
In August, Sony apologised to Rockstar after audio from the forthcoming title was leaked online.
Customers who had pre-ordered the game were allowed to download a "locked" version, but some were able to extract audio that revealed some of the plot lines.
The latest version of the best-selling video game replaces version 4, first released in 2008, with all the titles in the series selling more than 135 million copies.
A report in the Scotsman newspaper suggests the development costs for GTA5 could be in the region of £170m ($265m; 203m euros, making it one of the most expensive video games ever made.
The report anticipates that GTA5 could rake in £1bn in sales over its lifetime.
Such speculation has helped push the share price of Rockstar's owner Take-Two Interactive Software up by 65% over the past year.