Robert Kubica is not an option to make a one-off return with Toro Rosso because of an issue with his insurance, according to Auto Motor und Sport.
Following Carlos Sainz's early move to Renault, Toro Rosso are set to see the season out with Daniil Kvyat returning alongside the driver who replaced him: Pierre Gasly.
But the United States Grand Prix next weekend clashes with the Super Formula finale, a series where Gasly finds himself just half a point off leader Hiroaki Ishiura.
While Toro Rosso wants him to focus on his Formula 1 commitments, new engine suppliers Honda are eager for him to compete for the Super Formula title instead.
This ongoing dispute means there is potentially an open seat at Toro Rosso in Austin, Texas, with Kubica thought to be one option.
However, that does not appear likely after it has emerged that it would trigger a reimbursement of a substantial lifetime insurance payout Kubica received for being permanently unable to race in F1 following his horrific rally accident.
A full-time return, however, is more financially viable and new manager Nico Rosberg has said that is Kubica's main and only priority.
"If you are thrown into deep water in a strange car without a test, you can only lose," said Rosberg.
"It is better to keep going for the Williams opportunity."
Kubica will test alongside Paul di Resta for Williams at the Hungaroring before the end of the month.
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