Jenson Button is to partner Fernando Alonso at McLaren next season.
The team are to 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 after weeks of leaning towards Kevin Magnussen.
Button, the 34-year-old 2009 world champion, will continue into a 16th season and has his wish granted to test himself against the man widely regarded as the best in F1.
McLaren signed Alonso some weeks ago, but had been vacillating over the identity of his team-mate.
Chairman Ron Dennis had been keen to retain Magnussen, 22, but the continuing delays over making a decision have tipped the balance back in favour of Button.
The two were evenly matched for pure pace in 2014, with Button out-qualifying the novice 10-9 in 19 races.
The Englishman scored more than twice as many points as Magnussen over the season.
McLaren had intended to retain Magnussen 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 over Dennis's position has influenced the team to go the other way.
Magnussen may be retained as reserve driver for 2015.
McLaren, as well as representatives of both Button and Magnussen, were unavailable for comment on Wednesday.
Magnussen and Button have remained on good terms despite being pitted together for one seat at McLaren
The farrago over the identity of the team's second driver has detracted from the impact of McLaren's signing of double world champion Alonso.
This is quite a coup considering his standing in the sport and the fact McLaren have succeeded in luring him away from Ferrari.
Ferrari had their least successful season for 21 years in 2014 but still finished ahead of McLaren, largely thanks to the efforts of Alonso.
But the Spaniard was convinced by the arrival of new engine partner Honda and a restructuring of the team, including the signing of key personnel, that they were more likely to give him the chance 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.