RML, in association with AD Group, entered the international sportscar arena in late 2003 with an ex-works MG Lola EX257 in the inaugural race of what was then called the Le Mans Endurance Series, or LMES.

Over the next five years the team remained loyal to the MG brand, actively developing the chassis in partnership with Lola Cars of Huntingdon, and winning the LMP2 class in the Le Mans 24 Hours two years in a row; 2005 and 2006. Throughout this period the regular driver pairing of Thomas Erdos and Mike Newton established a reputation as front-runners in LMP2 prototype racing, adding the 2007 Le Mans Series drivers' title to their haul of race wins and lap records.

At the same time, the cars they drove were enhanced and developed, and underwent a series of name-changes too. The AER-engined MG EX257 was replaced by the EX264, initially with Judd V8 power (2005), and then reverting to the turbocharged two-litre AER engine for the 2006 season. Further development allowed the EX264 to complete a second season in 2007, which netted the Le Mans Series LMP2 titles not only for the drivers individually but also the team as a whole. A comprehensive upgrade for 2008, including updated MG XP-21 engine, resulted in re-homologation of the chassis as the MG EX265. This culminated in the MG marque's last-ever appearance in top level motorsport when the MG EX265C coupé made its debut (and, it transpired, one-and-only race) at Silverstone in September 2008.

With the MG-derived engine reaching the end of its development potential, and with no sign of the promised support coming from MG's new owners in China, a line was drawn under what had been a remarkable period of success for RML AD Group in endurance racing. The signing of a new engine and technical support deal with Mazda at the end of 2008 heralded the dawn of a new era, and in February 2009 the new Lola Mazda took to the track for the first time at Snetterton.