The move follows the success of Beijing's Olympic swimming team which won six medals, headed up by double gold-medallist Rebecca Adlington.
It has been a good week for British Swimming's hunt for backers. Earlier this week Speedo announced it was extending its current deal to 2016.
The BG funds will be broken down into a 50:50 split between elite athletes and grass roots swimming.
However other sports are struggling to obtain funding in the run-up to 2012, due to a shortfall in the government's targets for private sponsors. Part of the BG money will support water polo after the sports £50m funding was cut in half by UK Sport.
The government is trying to make up the shortfall in funding for 2012 teams with a variety of initiatives including allowing small businesses sponsorship opportunities.
Success at the Beijing Olympics has been an important factor in sports securing funding for London 2012 and beyond. The British Cycling team, the most successful British sport in Beijing
Olympics
British Gas signs £15m British Swimming sponsorship
LONDON - British Gas (BG) has agreed a 拢15m deal to back British Swimming with a sponsorship running through to 2015.