Jake Byrne topped the first Walter Hayes Trophy Heat qualifying, which was delayed by half an hour due to heavy rain.
Once the rain had died down the cars were allowed on to the Silverstone National circuit, which was still coated in a thick layer of water.
Matt Round-Garrido initially led the way in his B-M Racing Medina Sport JL17, and was rewarded for his bravery with the top spot for much of the session.
As the weather improved and the water was displaced, more drivers started to take risks, and the times dropped quickly.
James Roe Jr, in a Van Diemen, was first to beat Britcover National with Avon Tyres frontrunner Round-Garrido, who was shuffled down another spot when Ray driver Jake Byrne set a time two tenths faster than Roe’s.
James Raven, entering his first race for over a year, was an impressive fourth, ahead of James Scott-Murphy, Thomas Capezzone, Matthew Cowley and Michelangelo Amendola, who will be competing in the MRF Challenge later this month.
Ed Thurston was fastest of the Pre93 runners in 10th, with Neil Hunt the fastest Historic runner in 17th.
William Nuthall brought out yellow flags late in the session when he spun his Jamun T2 on the pit straight.