Simon Cowell put £250,000 towards the £500,000 winner's total upon his return to the panel last year, while it was £100,000 previously.
However, producers have decided to cut the prize to £250,000, with Cowell not putting forward his own money, according to The Sun.
"Half a million was a massive prize - one of the biggest on telly. But it was only supposed to be for one year," a source explained.
The eventual winners will still receive a slot at the 2013 Royal Variety Performance.
Show insiders insisted that the prize and publicity from the show will be "life-changing".
One said: "BGT is a massive platform for the acts. Some of them are carving out careers, but the show turns them into household names.
"Look at Susan Boyle. She was desperate for a break. The show gave that to her, and now she is worth millions of pounds."
Ashleigh & Pudsey won the 2012 series, while the show's first winner Paul Potts has made around £6 million during his opera career.
James Corden will play Potts in a forthcoming biopic.
In pictures - Britain's Got Talent 2013 episode two: