Update 1.20pm: Britain's Paul di Resta will make a shock return to Formula One at the Hungarian Grand Prix after Felipe Massa was ruled out through sickness.
Massa, 36, had been given the all-clear by the FIA to continue his participation this morning after he was taken to hospital following complaints of dizziness.
But the Brazilian had to cut short his run in final practice and completed just 12 laps after falling unwell again.
Williams reserve driver Di Resta, who last raced in the sport back in 2013, has not tested this year's car, and will be thrown into the deep end in qualifying later today.
"After feeling unwell during practice in Budapest on Friday and Saturday, Felipe Massa will not drive for the remainder of the Hungarian Grand Prix weekend," Williams said in a statement released ahead of qualifying.
"Felipe visited the medical centre and the MH EK Honvedkorhaz hospital for precautionary tests on Friday, after feeling unwell and dizzy during FP2. He was cleared to take part in Saturday's practice session by the FIA medical delegate, but he felt unwell again during FP3 and has made the decision to withdraw from the weekend.
"Williams supports Felipe's decision and the team will work with him to ensure he makes a full recovery, with a view to return to the race track for the Belgian Grand Prix.
"Following this decision, the team's Reserve Driver Paul di Resta will drive alongside Lance Stroll for the remainder of the Hungarian Grand Prix weekend."
Felipe Massa has been given the all-clear to take part in the Hungarian Grand Prix despite falling unwell yesterday.
Massa, the 36-year-old Williams driver, complained of feeling dizzy after practice at the Hungaroring, and was taken to the on-track medical centre.
The Brazilian was then transferred to the Honvedkorhaz hospital in downtown Budapest for further tests before he was given the green light by the FIA to continue his participation here this weekend.
He will however, be subjected to a further medical examination after final practice which takes place later this morning.
"Felipe was feeling unwell and dizzy after FP2, so went to the hospital purely as a precaution to check he was OK," Massa's Williams team said in a short statement.
"He has seen the FIA medical delegate at the circuit this morning who is happy that he is fit to continue driving this weekend.
"They will do a further routine check after FP3 to ensure he is fine ahead of qualifying."
Scot Paul di Resta, who last raced a Formula One car back in 2013, is the reserve driver for Williams and had been placed on standby by the British team before Massa was cleared to continue.
Di Resta, 31, is also due to deputise for pundit Martin Brundle in the Sky Sports commentary box in Hungary. Brundle is absent from this race after he was taken ill moments before the British Grand Prix a fortnight ago.