The ads promote the energy supplier's new fixed-price offer until April 2007. The new work takes a softer approach than the blokey tone set by the Tomlinson campaign.
The spot, which was developed by Clemmow Hornby Inge, comes after the company lost more than 900,000 customers last year, with more than two-thirds departing following a 21 per cent price increase in September.
Blethyn, who starred in the films Little Voice and Mike Leigh's Secrets and Lies, takes over from Tomlinson as the supervisor at British Gas' training academy. She uses a wheel-of-fortune gameshow theme to show new recruits that rival suppliers have uncertain pricing.
She takes a less aggressive tone than Tomlinson with the recruits, whose female numbers have been bolstered.
Nick Smith, the director of marketing and strategy at British Gas, said: "Recruit-ing a new supervisor, Brenda Blethyn, to champion our first TV product campaign of 2005, will help us to extend our messages to a wide target audience group.
"This ad also helps draw attention to the growing number of female engineers graduating from our real-life engineering academies. In the past year, for example, we have doubled the number of our female engineering recruits."
The ad, which breaks on 24 January, will continue to use the "doing the right thing" line that was launched in 2003.