Jenson Button is to partner Fernando Alonso at McLaren next season.
The team will announce their driver line-up at a news conference at their factory in Surrey on Thursday.
But BBC Sport has learned they have decided to retain Button, world champion in 2009, after weeks of leaning towards Kevin Magnussen.
The news means Button, 34, will extend his F1 career into a 16th season and team him up with the driver widely regarded as the best in the sport.
McLaren signed Alonso, 33, some weeks ago but had been vacillating over the identity of his team-mate.
|McLaren's 2015 partnership|
|Fernando Alonso||Jenson Button|
They had intended to retain Magnussen, 22, on the basis that the team's data had suggested he was fractionally quicker and had youth and promise on his side.
But Button's strong finish to the season, his experience and perhaps unrest at boardroom level influenced the team to go the other way.
The two drivers were evenly matched for pure pace in 2014, with Button out-qualifying his Danish team-mate 10-9 over the course of the season.
The Englishman also scored more than twice as many points.
Magnussen, who ended with 55 points compared to Button's tally of 126, may be retained as reserve driver for 2015.
The farrago over the identity of the team's second driver has overshadowed McLaren's signing of double world champion Alonso.
The Spaniard's capture is quite a coup considering his standing in the sport and the fact he was at Ferrari.
Ferrari may have endured their least successful season for 21 years, but they still finished ahead of McLaren in 2014, thanks largely to the efforts of Alonso.
McLaren's new engine partnership with Honda helped convince Alonso to rejoin them, as did the signing of key team personnel.
Alonso felt McLaren were more likely to provide him with the opportunity to secure the third title he has been seeking since he won his second in 2006.
Alonso is reputed to have signed a deal worth $40m (£25.5m) a year, which would make him the best-paid driver on the grid.
His contract is believed to be for two years with the option for one more, but Alonso will almost certainly be able to leave at the end of 2015 if McLaren's performance is not up to expectations next season.
Button will be on a one-year deal at a salary reduced by at least half from the reputed £12m he was earning in 2014.
The 2015 F1 grid
Mercedes - Lewis Hamilton and Nico Rosberg
Red Bull - Daniel Ricciardo and Daniil Kvyat
Williams - Felipe Massa and Valtteri Bottas
Ferrari - Sebastian Vettel and Kimi Raikkonen
McLaren - Fernando Alonso and Jenson Button
Force India - Nico Hulkenberg and Sergio Perez
Toro Rosso - Max Verstappen and Carlos Sainz Jr
Lotus - Pastor Maldonado and Romain Grosjean
Sauber - Felipe Nasr and Marcus Ericsson