Talks between Toro Rosso and Honda about a Formula 1 engine supply deal are progressing, following a meeting between Red Bull's Helmut Marko and Honda chief Masashi Yamamoto on Tuesday.
Honda is keen to take on a second team, following the cancellation of its deal with Sauber, as it feels that extra data gathered would help its development rate with McLaren.
Toro Rosso has emerged as the leading candidate, even though the Italian team has a deal with Renault in place for next year.
Autosport understands Tuesday was the latest in a series of meetings between Red Bull motorsport advisor Marko and Yamamoto, who is responsible for Honda's motorsport budget and reports directly to the board, in recent weeks.
Sources with good knowledge of the situation say talks have focused around Toro Rosso taking a Honda supply from 2018, with Marko interested in the senior team Red Bull switching in the future if Honda improves its performance sufficiently.
In 2015, Red Bull wanted to pull out of its deal with Renault and spoke to Honda, as well as Mercedes and Ferrari, about an engine supply, but that move was blocked by then-McLaren boss Ron Dennis.
With Red Bull keen to have a works supply, tying up with Honda in the future would be a solution if McLaren extricates itself from its deal with Honda and finds an alternative.
Should Toro Rosso and Honda come to an arrangement for next year, it would also need to source a gearbox contract.
Autosport understands McLaren could be in position to supply Honda with a gearbox that the Japanese manufacturer could then offer as a package with its engine to Toro Rosso.
As any Toro Rosso agreement would effectively replace the Sauber customer deal that fell through, it is believed McLaren would not stand in the way.
But it is understood McLaren would not be keen for Red Bull to get the engine and would consider vetoing such an arrangement again.
McLaren's deal with Honda continues into next year but it is still evaluating its options following poor performance - even though Ferrari and Mercedes have made it clear they will not supply engines to the team.