Isaac Nader produced a devastating attack in the home straight at the National Stadium in Tokyo to snatch victory in 3min 34.10sec, just two-hundredths of a second ahead of 2022 world champion Wightman.

Kenya's Reynold Cheruiyot took bronze in a race in which Britain's defending champion Josh Kerr, one of the pre-race favourites, came in last, carrying an injury.

"In the last 100 I fully believed in myself. I knew Jake would be fighting until the end so I went all in for a dive," said Nader.

"This was probably the first time I did it in my career but I couldn't risk losing a world title. Clearly, it paid off in the end."

Nader, 26, who was born in Faro to a Moroccan father and Portuguese mother, said he was now aiming for more gold medals.

"Next year I will go into the European championships as the world champion, and for the world championships the year after (in Beijing in 2027), so I will pace myself and take each event as it comes."

Wightman said the "silver is a gold for me" after he endured a torrid time with injuries since he too pulled off a shock world title win in Eugene in 2022.

"I knew I still had something in me," he told BBC. "I knew I was going to run to try and win today and whatever happened, I would be happy.

"When you get that close you cannot help but think what if I had done more? But I left everything out there."

Wightman added: "It has been a few bleak years so making this team was the main thing.

"I believe that you get what you pay for in the end. I am so happy that all the persistence has been worth it.

The field was blown wide open in the build-up to the final when Norway's 2021 Olympic champion Jakob Ingebrigtsen failed to make it out of the heats.

Ingebrigtsen sustained an Achilles injury after sweeping to a world indoor double in March, and a first outdoor outing in Tokyo proved too much for him.

Then Olympic champion Cole Hocker was disqualified in the semi-finals on Monday for 'jostling'.

In the final, Niels Laros of the Netherlands went straight out to the front of the pack, Wightman on his shoulder, with Kerr tucked into the middle of the 14-strong pack.

The Kenyan Cheruiyot duo, Timothy and Reynold, moved to the front, the former injecting a sudden surge that saw the pack split.

Kerr suddenly began to struggle with 600m to run, for a moment skipping along after seemingly suffering a leg injury.

Timothy Cheruiyot took the pack through the bell, with Kerr now well adrift in last position but Wightman made his move heading into the final 200m, rounding the Kenyan and Laros.

The Briton looked like he had done enough for an improbable second world gold, but he had not counted on Nader's perfectly-timed finish.

The Portuguese, whose only previous medal showing was a bronze at the European indoors in Apeldoorn in March, dipped expertly at the line in a thrilling finish as both Portugal and Britain picked up their first medals of these world championships.