McLaren has shared reserves with Mercedes since the start of the 2021 season instead of having its own dedicated driver in the role, making Stoffel Vandoorne and Nyck de Vries their primary two options if required. Ex-Force India driver and Sky F1 pundit Paul di Resta has also been a further back-up if required.
But Alpine has announced that Piastri has now also joined the pool of drivers available to McLaren if required.
"In the spirit of offering maximum racing opportunities to our stable of young talent, we have agreed that McLaren may call upon BWT Alpine F1 Team reserve driver Oscar Piastri in the event one of their drivers is unable to race," a statement from Alpine reads.
"Oscar will remain fully contracted to Alpine, and we will retain first option on his services."
The news comes after McLaren revealed on Friday that Daniel Ricciardo had tested positive for COVID-19, ruling him out of the pre-season test running at the Sakhir track.
Ricciardo is now isolating, but said that he was feeling better and would "just focus on next weekend" when the F1 season gets underway, while McLaren team boss Andreas Seidl expressed his confidence that Ricciardo would be able to return and race in Bahrain.
The team has also made clear the deal with Piastri is not related to Ricciardo’s situation.
"I'm confident he will be fit again next weekend," Seidl said.
"He is feeling better already. But as we know, it simply takes a few days until you are fully back in good shape.
"Obviously in terms of reserve drivers, it is similar to previous years. We have an agreement with Mercedes to share reserve drivers. We have also Paul di Resta, similar to last year, on standby in case it is needed.
"But again, expect Daniel to be back in good shape next week."
Prior to Piastri's announcement in the role, de Vries would have been the only driver available to McLaren next week were Ricciardo to be ruled out, as both Vandoorne and di Resta are at Sebring for the IMSA/World Endurance Championship double-header next week.
De Vries will be on-site in Bahrain next weekend, and Mercedes has first option on his services if required at all.
Ricciardo's inability to test in Bahrain this week meant Lando Norris completed all of McLaren's running over three days. The Australian driver will therefore head into the new season with just one-and-a-half days of running under his belt, completed in the Barcelona test.
"In terms of preparation going into the season, obviously, that's not what you're aiming for," said Seidl.
"Definitely it puts us and Daniel backwards as well but in the end, it is something we couldn't control and Daniel couldn't control. We simply have to accept that it is.
"With the experience Daniel is having, going also now into the second year together with the team, [I'm] still optimistic that the small disadvantage he will have starting the race weekend it doesn't take that long until he is fully back."
"It would be great if Oscar can get some races under his belt in F1 and get that very valuable experience," Alpine F1 team boss Otmar Szafnauer said on F1 TV.
"We've talked to McLaren about Oscar being a reserve if they need him. He’s definitely our reserve first and foremost, so if there was a conflict, we would take him.
"But if Daniel can’t drive, then we’ll offer him up.
"I wish Daniel a speedy recovery, I would think six days from now, I don't know when he contracted Covid, but in 7-10 days you get rid of it and you're fine."
