The energy supplier announced it had dropped it prices by 7%, making it the first major provider to pass on savings after a industry-wide drop in wholesale prices in 2009.
The "price drop" ads, created by CHI and Partners, includes TV, outdoor, radio and print activity.
Media buying has been handled by Aegis Media's Carat and includes radio coverage on Absolute Radio and a cover wrap on free urban newspaper Metro, published in cities nationwide today.
A TV ad has also launched across terrestrial schedules this morning and there are full page ads running in The Independent, The Daily Mail, The Daily Telegraph and The Times.
Digital and static posters in bus shelters are also being rolled out across the country while British Gas has erected marketing stands in 26 mainline railway stations.
The price cut, which will benefit 8 million households, takes effect immediately and makes British Gas, on average, the cheapest supplier of standard gas and electricity across Britain. The change is expected to save the average gas customer £55 a year.
This is the third time British Gas has cut prices in the past 12 months - cutting a total of £187 off the average annual dual fuel bill.
Last year, the company cut both standard gas and electricity prices by an average 10%.
Announcing the gas price cut, British Gas managing director, Phil Bentley, said: "I'm pleased we're able to offer our customers some extra help with this gas price cut - and that we're able to do this while it's still winter, allowing our customers to really feel the benefit.
"This latest price cut means that, no matter where you live in Britain, British Gas is offering on average the cheapest standard gas, electricity and dual fuel prices - beating all other major suppliers."
Last summer, British Gas attacked rival EDF Energy's 'Green Britain Day' with an ad stating 'Green Britain Day. For us, it's everyday.'
The ad ran in the national press a day before EDF's event with the strapline: "British Gas is thinking about new ways to be green every single day."
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