The plan to ban tyre warmers in 2015 is currently in doubt.
This week at the Bahrain test, after Williams test driver Felipe Nasr tried an experimental Pirelli tyre, the Brazilian said it was "very difficult to get them up to temperature, even in the Bahrain heat".
"It will be even more of a challenge at tracks such as Silverstone or Spa," he warned.
Pirelli boss Paul Hembery said the main issue is tyre pressure, as the difference between a cold and a hot tyre is significant.
"At present," he is quoted by Speed Week, "we believe it will be difficult to implement the FIA's plan for 2015."
F1's governing body wants to get rid of expensive tyre warmers...
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