INDIANAPOLIS -- The Indy Racing League struck another major blow in its battle with CART for supremacy of American open-wheel racing, announcing Thursday that Honda will join the series in 2003 as a fourth engine manufacturer.

Honda has built turbocharged engines for Championship Auto Racing Teams since 1994, but the company decided in October that this would be its last year after CART adopted a formula similar to the IRL's normally aspirated V-8 machines.

''It's huge,'' said Michael Andretti, whose team is using Honda engines in CART this year. ''That's a big feather in the IRL's cap, for sure. Honda has been such a big supporter of American racing.''

Initially, Honda planned to drop out of American open-wheel racing altogether, then reversed course by reaching a stunning partnership with Ilmor Engineering Inc. to enter the IRL for the first time.

Tony George, president of the IRL and the Indianapolis Motor Speedway, was asked if he had driven the final nail in CART's coffin.

''I bring my hammer to work every day,'' he said, smiling. He quickly added, ''I mean, we're trying to build our series every day.''

The deal was announced at the speedway shortly after the final practice for Sunday's Indy 500.

''We very much look forward to coming here next year and winning,'' said Robert Clarke, general manager of Honda Performance Development, the Japanese carmaker's racing subsidiary.

Later Thursday, the IRL announced it has signed a three-year agreement to race at Twin Ring Motegi, Honda's racetrack north of Tokyo. Motegi has been the site of a CART race for five years and ran the last race of its current contract at the 1.5-mile oval last month.

It will be the IRL's first race outside the United States.

''We always said if the right opportunity came along, we'd do it,'' George said. ''Basically, this is still an American-based, open-wheel, oval series. We'll stick to what we do, but there may be a chance to take advantage of one or two outside opportunities. Maybe a road course.''

Given the tight time frame for developing an engine, Honda had to work out the partnership deal with Ilmor to be ready for the 2003 season. Design of the new engine has already started, with actual testing scheduled to begin Sept. 1, according to Clarke.

Toyota, which also has furnished engines for CART for several years, already announced it will join the IRL next season. Company officials said a decision on remaining in CART will be made in the next few weeks.

Chevrolet (formerly Oldsmobile) and Infiniti already are building engines for the 7-year-old series, which has left once-dominant CART struggling for its very survival.

The older series has only one minor manufacturer lined up to produce engines for 2003, prompting speculation that major CART teams such as Chip Ganassi Racing and Team Green will be moving to IRL next season. Roger Penske made the jump this year, and eight CART regulars are racing in the Indianapolis 500, the IRL's premier event.

Team Green has three Chevy-powered cars in Sunday's field -- driven by Andretti, Dario Franchitti and Paul Tracy -- but it uses Honda engines when competing on the CART circuit, making it a logical starting point when the carmaker begins lining up its IRL clients.

''We are certainly in the middle of working on next year's program,'' said Kim Green, who runs Andretti's operation and is the brother of Team Green owner Barry Green. ''All I can say is we will be involved in motorsports.''

A.J. Foyt, a former CART team owner who jumped to the IRL when it was founded, gloated about Honda's announcement. He has long maintained that George would win the open-wheel war because holds the trump card -- Indianapolis Motor Speedway.

''Let's face it,'' Foyt said. ''No one would know me or Mario Andretti or Roger Penske if it wasn't for this place. This place is like the Kentucky Derby. You've got the Preakness and the Belmont, but they're not the Derby.''

The deal with Michigan-based Ilmor marks the first time Honda has agreed to take on a partner in its racing program.

''It does seem strange, doesn't it?'' Clarke said. ''We've prided ourselves on not having that kind of relationship before. But it boiled down that this was our only choice.''