The Mercedes and Red Bull drivers had been locked in a tight fight with McLaren duo Daniel Ricciardo and Lando Norris for the top four positions at Monza during the early phase of the race.
Following the first round of stops, with Ricciardo leading, Hamilton emerged from the pits in a net third just ahead of Verstappen as they went down to the first chicane on lap 26.
Verstappen tried to use his momentum around the outside of the right-hander Turn 1 and, as Hamilton moved to squeeze Verstappen towards the kerb to protect his position, the pair came to blows at the Turn 2 left-hander.
Verstappen – who had been delayed by a slow 11-second pitstop – appeared to bounce over a kerb at the apex of the corner and was then launched up into the air – landing on top of Hamilton’s car as the pair then came to rest in the gravel.
Verstappen quickly came over the team radio and said: “That’s what you get when you don’t leave the space.”
The FIA announced that the incident would be investigated after the race, with both drivers likely to be summoned to see the stewards.
Lewis Hamilton, Mercedes, climbs out of his car after crashing out with Max Verstappen, Red Bull Racing
Photo by: Zak Mauger / Motorsport Images
On lap one of the race, Hamilton and Verstappen had already nearly come to blows when the pair battled wheel-to-wheel over second position at the second chicane.
Hamilton was forced across the run-off area, dropping behind Norris, and complained that he had been left no space.
Their race-ending clash brought out the safety car, with Ricciardo leading the race from Charles Leclerc’s Ferrari and Norris in third.
Ricciardo went on to win, leading a McLaren 1-2 ahead of Norris, with Valtteri Bottas completing the podium in the second Mercedes that had started the race last due to a Friday power unit change.
The Italian GP crash between Verstappen and Hamilton comes after the pair controversially came to blows on the opening lap of the British GP, the Dutchman fired into a 51g impact at the high speed Copse corner.
Hamilton was deemed to have been predominantly to blame and served a 10-second penalty, but still went on to win the race with a late pass on Leclerc.