The Canadian Grand Prix has been red-flagged after 25 laps due to heavy rain at the Circuit Gilles Villeneuve.
Rain throughout the day in Montreal had seen the race start behind the Safety Car before the cars were released to begin racing on lap five.
Sebastian Vettel maintained his lead despite a poor restart that briefly allowed Fernando Alonso to get a run into turn one, although the Ferrari failed to get ahead.
Vettel stayed out front while behind there was drama as Lewis Hamilton tipped Mark Webber into a spin at turn one and was then forced to retire after clashing with McLaren team-mate Jenson Button on the start-finish straight while trying to come back up the order.
The incident forced the safety car to be deployed as debris was removed from the circuit.
Improving conditions saw a number of drivers - including Button, Alonso and Mercedes pair Nico Rosberg and Michael Schumacher - pit for intermediate tyres and Button was soon the quickest man on track.
However, another sudden downpour saw those drivers on Intermediates forced to dive back into the pits while the Safety Car was sent out, with Vettel, Mark Webber and Felipe Massa electing to make what was effectively a free stop for fresh wet tyres.
With rain getting heavier, the race has now been stopped with Vettel leading from Kamui Kobayashi, Massa, Nick Heidfeld, Vitaly Petrov and Paul di Resta.
Of the top six, Vettel and Massa are the only drivers to have made a stop.