circuit, 2008 world champion Hamilton set the pace with a largely meaningless time of one minute 56.345 seconds after lunch.
Lewis Hamilton is looking for a repeat of his 2008 British GP victory that came in torrential conditions at Silverstone. Getty images
Frenchman Romain Grosjean had been fastest for Lotus in the morning session in 1:56.552.
Waterlogged campsites led to monster tailbacks outside the circuit as organisers struggled to get vehicles parked on firm ground with 80,000 spectators expected on one of the wettest days of the summer so far.
Many ticketholders were reported to be still locked in jams outside, with little hope of seeing any of the day's action, even as the second session started.
Despite the delays and frustration, there was no denying the enthusiasm of those who made it in.
The grandstands looked fuller than on race day at some sunnier circuits, even if drivers rewarded them with only tentative laps on the glistening track.
"Bit of swimming today only 6 laps in FP1! UK weather is just unreal," declared Marussia's German driver Timo Glock on Twitter after the first session.
Staying on the asphalt was tricky, with the afternoon practice halted for 10 minutes when Brazilian Bruno Senna veered off into the barriers after his Williams hit standing water.
Senna, who has yet to race at Silverstone in Formula One, was unhurt but the car looked heavily damaged.
Ferrari's Fernando Alonso, the current championship leader, also hit the barriers after a similar off-track excursion late in the session.
Toro Rosso's Australian Daniel Ricciardo was second on the morning timing screens ahead of Hamilton.
Hamilton, whose 2008 victory makes him the last British driver to win his home race, completed only six laps in the first 90 minute session and eight in the afternoon.
"The conditions are pretty dodgy to say the least," said Red Bull's Australian Mark Webber, who did seven laps in the morning and was sixth fastest. He did not do a timed lap in the afternoon.
"We don't want to do a thousand laps today because the forecast looks pretty dodgy for the weekend and we need these tyres," he said.