McLaren has struck a deal with Renault for the remainder of 2019 whereby it can call upon the services of Sergey Sirotkin if either Carlos Sainz or Lando Norris become unavailable.
However, Renault will retain a priority right over the Russian in case it also needs to replace one of its works drivers.
"McLaren and Renault have reached an agreement for McLaren to have access to our Third and Reserve driver Sergey Sirotkin," Renault said in a statement.
"If Carlos Sainz or Lando Norris are unable to complete a full race weekend in 2019. Sergey remains our official Reserve, but McLaren can call upon his services this season if needed."
Sirotkin endured a difficult rookie year in F1 last season, scoring just a single point with Williams, before the 23-year-old was ousted from the beleaguered British outfit to make way for Robert Kubica.
In addition to his involvement with Renault, Sirotkin is also racing this season in the WEC with LMP1 squad SMP Racing.
"McLaren is a legendary team and I’m happy to have this opportunity to work with them, join their history, see from the inside how the team and drivers work," Sirotkin said. "I also thank SMP Racing and Renault for their support."
"McLaren team principal Andreas Seidl added: "This is an expedient solution for McLaren in 2019. Sergey is a known quantity, has the necessary super licence points and is formally aligned to our engine provider, Renault, so he is the obvious candidate."