Green Flag had been the main sponsor of the popular saloon car series since 2002 and formed the relationship with BTCC following the success of its title sponsorship of the British Formula 3 championship from 2000 to 2002.
Green Flag made its decision after "a review of its brand positioning programme. The company said that "it has more than maximised its return on investment from the sponsorship deal in terms of brand and name awareness after three fruitful years and is currently exploring new opportunities."
Marketing manager Hillary Williams said, "Our sponsorship of the BTCC has been an unqualified success and our relationship has been both long-standing and highly valued. Our three-year association with the UK's most prestigious saloon car championship has taken place during a period that has seen a rise in the profile of BTCC as well as the Green Flag brand.
"I am pleased that we have been able to contribute to the revival of the BTCC series, which now enjoys excellent TV coverage, growing crowds and continues to attract new teams and drivers."
BTCC series director Alan Gow said, "I would like to take the opportunity of thanking Green Flag for their contribution to the growing success of the BTCC and I am delighted that their involvement met or exceeded all of their targets. It's another great testament to the power of the BTCC."
James Thompson, won last year's Green Flag BTCC, in a thrilling contest with Vauxhall team mate Yvan Muller. One of the most exciting championships in years was decided at the last race of the season at Donington Park. The 2005 MSA British Touring Car Championship begins at Donington Park on 9 and 10 April.