Brendon Hartley (right) made his Formula 1 debut at last weekend's American Grand Prix

Pierre Gasly and Brendon Hartley are poised to form the Toro Rosso driver line-up in Formula 1 in 2018.

Team boss Franz Tost said there was a "high possibility" the Frenchman and New Zealander would stay on next year after being drafted in late this season.

And the Austrian confirmed Russian Daniil Kvyat, who has raced in F1 for one of the Red Bull teams since 2014, had been dropped from their programme completely.

Gasly and Hartley have in total so far raced in only three grands prix.

Gasly, 21, made his debut in Malaysia three races ago after Kvyat had been dropped from the Toro Rosso team following the Singapore Grand Prix in September.

Hartley, 27, then made his debut in the US race last weekend as a replacement for Gasly who was due to race in the Japanese Super Formula series while Kvyat made a one-off return as a replacement for Carlos Sainz, who has moved to Renault on loan.

Gasly returned to Toro Rosso for this weekend's Mexican Grand Prix in place of Kvyat, who has been told he has no future with Red Bull.

"He is not any more with Red Bull and he is therefore free to do whatever he wants to do," Tost said.

Gasly's presence in Toro Rosso next season is not a surprise - he is next in line in the Red Bull junior programme.

But Hartley's rehabilitation is a remarkable turnaround for someone who was dropped from the Red Bull junior line-up back in 2010 after spending 18 months as reserve driver for the senior team.

Pierre Gasly made his Formula 1 debut with Toro Rosso in Malaysia

Hartley has since rebuilt his career in the world endurance championship, where he was champion in 2015 and has also won the Le Mans 24 Hours, and contacted Red Bull motorsport manager Helmut Marko when his Porsche team announced it would quit the series at the end of this season.

Tost said: "Brendon is a very high-skilled driver, he is very competitive. I am really happy that he is back and I can tell you we give him a competitive car, he will be there and he will fight in F1 for success.

"I hope next year we will bring together a competitive package that he can fight for victories and other positions."

Kvyat spent a year at Toro Rosso in 2014 before being promoted to the senior team in 2015 after Red Bull's four-time world champion Sebastian Vettel moved to Ferrari.

But Kvyat failed to match up to team-mate Daniel Ricciardo and was dropped from Red Bull back to Toro Rosso after just four races of the 2016 season and replaced by Max Verstappen.

After being out-performed by Sainz for nearly two seasons, Red Bull have now decided the 23-year-old does not have what it takes to become a top-line F1 driver.