Helmut Marko has said that he is putting together a "long list" of potential Toro Rosso drivers for 2019.

Pierre Gasly is being promoted to Red Bull Racing, while Brendon Hartley has underperformed this year.

"There needs to be a significant increase (from Hartley) in the last races this season," Marko told Speed Week.

The 75-year-old confirmed that Hartley will not be ousted within 2018, but it means that Toro Rosso could end up with two free seats for next season.

Red Bull junior Dan Ticktum is struggling to secure the necessary super license, while McLaren says that the axed Stoffel Vandoorne should be considered.

Also mentioned as a candidate by Christian Horner is former Red Bull and Toro Rosso driver Daniil Kvyat, while rumours have linked Jean-Eric Vergne and even Robert Kubica and others with the seat.

"There is no shortlist, it's more like a long-list," Marko added. "There are ten names on it."

Esteban Ocon was seemingly ruled out, however, with Marko noting that one driver "with a Mercedes contract" will "probably not" be considered.