McLaren have "serious concerns" over whether they can win the world championship with engine partner Honda, says executive director Zak Brown.
The former world champions are facing their worst ever season after failing to register a single point so far in 2017 after a series of engine failures.
Brown said engine upgrades promised for Sunday's Canadian Grand Prix were not ready and the team is "near our limit".
"Honda's working very hard but they seem a bit lost," he told Reuters.
"We were eagerly awaiting this upgrade as were our drivers and it's a big disappointment that it's not coming.
"It's not lack of effort, but they are struggling to get it to come together."
Brown, who replaced Ron Dennis at the McLaren helm last year, added: "Our preference is to win the world championship with Honda.
"But at some point you need to make a decision as to whether that's achievable. And we have serious concerns.
"Missing upgrades, and upgrades not delivering to the level we were told they were going to, you can only take that so long. And we're near our limit.
"We're not going to go into another year like this, in hope.
"There's lots of things that go into the decision and we're entering that window now of 'which way do you go when you come to the fork in the road'."
McLaren's renewed partnership with Honda in 2015 was billed as a return to the glory days of their collaboration in the late 1980s and early 1990s, when Alain Prost and Ayrton Senna dominated.
Yet McLaren remain without a race win since 2012 and their best finish this season was when Fernando Alonso, who started seventh on the grid, came 12th in last month's Spanish Grand Prix.
In March it was revealed McLaren had made an exploratory approach to Mercedes about engine supply in the wake of problems with Honda.
And Brown again raised the prospect of McLaren paying for engines in future.
"Do I think you can win with a customer engine? I think you can," he said.
How to follow the Canadian Grand Prix on BBC Sport
|Canadian GP coverage details (all times BST)|
|Date||Session||Time||Radio coverage||Online text commentary|
|Thursday 8 June||Preview||21:00-21:55||BBC Radio 5 live, online and podcast|
|Friday 9 June||First practice||14:55-16:35||BBC Sport website||From 14:30|
|Friday 9 June||Second practice||18:55-20:35||BBC Sport website||From 18:30|
|Saturday 10 June||Third practice||14:55-16:05||BBC Sport website||From 14:30|
|Saturday 10 June||Qualifying||17:55-19:05||BBC Sport website||From 17:00|
|Sunday 11 June||Race||18:30-21:00||BBC Radio 5 live||From 17:30|
|Monday 12 June||Review||04:30-05:00||BBC Radio 5 live, online and podcast|