Lewis Hamilton took the extraordinary step of demanding an explanation from the stewards midway through Sunday's Italian Grand Prix after he was served a penalty which paved the way for Pierre Gasly to complete one of the most surprising victories in motor racing's recent history.

Gasly, the 24-year-old Frenchman unceremoniously dumped by Red Bull 13 months ago, celebrated his maiden Formula One win, with McLaren's Carlos Sainz, and Racing Point driver Lance Stroll completing an improbable podium at Monza. Hamilton finished seventh.

Hamilton had looked on course to race to his sixth win of the year after establishing a commanding 11-second lead with just 20 of 53 laps on the board. But the world champion and his all-conquering Mercedes team performed an uncharacteristic error.

With Kevin Magnussen's stricken Haas car parked up in a dangerous position, and the Safety Car subsequently deployed, Hamilton missed two red crosses to inform him that the pit lane was closed.

Hamilton and the Mercedes pit wall had 12 seconds to react to the order but failed to do so. The Briton dived in for fresh tyres and his fate was sealed.

The stewards hit Hamilton with a 10-second stop-and-go penalty for the infringement, ending his hopes of claiming a 90th career win and moving to within one of Michael Schumacher's victory record.

Charles Leclerc's staggering 180mph crash at the menacing Parabolica on lap 24 led to a red-flag period – and a frustrated Hamilton left his Mercedes in the pit lane, before walking up the staircase at the rear of the paddock and heading to the stewards' office.

"I wanted to see what had been missed because I swore there wasn't a red light at the entry to the pit lane," Hamilton later explained.

"The stewards showed me the on-board video and the signs which had a cross through them. I didn't see them because I was looking elsewhere.

"It feels severe. A stop-and-go penalty often comes if you have done something intentional, driving erratically or putting someone in danger.

"I don't know if they can make the rule better for the future, but I accept it and move forwards. I hold myself accountable."

Hamilton's mistake was the catalyst for the best race of the year, providing a shot in the arm for the sport after a series of predictable affairs.

Gasly started 10th but was third at the restart after he fortuitously pitted for tyres a lap before the Safety Car was released.

He then made mincemeat of Stroll when the race resumed on lap 28. Hamilton stopped for his penalty, Gasly assumed the lead and held his nerve. Sainz chased him all the way to the line but the Alpha Tauri driver triumphed by just 0.4 seconds.

Gasly, who became the first French driver to win a grand prix since the similarly unlikely triumph for Olivier Panis in a Ligier at Monaco 24 years ago, was reduced to tears when he crossed the line.

"Oh my God," he screeched over the radio. "What did we do!? We won the f****** race! Oh my God. Oh my God."

The victory is the silver lining on what has been a turbulent year and a half for Gasly, sent back to Alpha Tauri after a series of poor performances for senior team Red Bull. His childhood friend Anthoine Hubert was also killed in a Formula Two at Spa-Francorchamps in August last year.

"Honestly, it is unbelievable," Gasly said. "I cannot believe what is happening right now.

"It was such a crazy race. I have been through so much in the space of 18 months. I struggle to realise what has happened. I have got no words."

Despite Hamilton's troubles here, his 47-point championship lead remains intact after he passed eight cars and secured a bonus point for the fastest lap following his penalty. Max Verstappen retired with a broken Red Bull, while Hamilton's team-mate Valtteri Bottas finished only fifth following a poor start.

British driver Lando Norris was fourth on a strong afternoon for the resurgent McLaren team. But a miserable weekend was sealed for Ferrari when both cars failed to complete their home race.

Sebastian Vettel retired with a brake failure on lap six before Leclerc's seismic shunt. Leclerc went to the on-track medical centre but was given the all-clear.