Boris Johnson has admitted breaking the speed limit after he put the "pedal to the metal" when he tried out a new car on the first day of his trade mission to Japan.
London's mayor gave the Toyota Mirai hydrogen car "some welly" as he took it out for a couple of spins on the streets surrounding the firm's headquarters.
But after hitting 120 kilometres an hour (75mph) on one stretch during the test drive, the Conservative MP noticed the limit was just 40km (25mph) the second time round.
"I was well over the speed limit," he said. "Never mind."
A fleet of the zero-emission vehicles is being delivered to Britain under a deal announced by the mayor during the visit. Toyota is sending 12 of its Mirai, which means future in Japanese, with four destined for use on London's transport network and the rest going to private companies, including a taxi firm.
The cars are among the most technologically advance in the world, with a driving range similar to traditional petrol models despite being far more environmentally friendly.
Mr Johnson said: "It is fantastic that London will benefit from these new state-of-the-art hydrogen vehicles.
"By embracing this technology of the future, we aim to consolidate hydrogen's role as a practical alternative fuel for the 21st century and beyond.
"I am sure that Transport for London will provide the ideal environment for us to see everything the Mirai can do and, in doing so, take another great step towards improving air quality in our city and protecting the health of Londoners."
The Mirai, which emit only water vapours from the tailpipe, will be used by Transport for London for engineering work across the tube and bus network.
Mr Johnson earlier stunned locals when he danced in the aisles of a swanky department store.
He took hold of professional dancer Charlotte Gooch, who is starring in a touring production of Top Hat, and led her in a waltz in Hankyu, Osaka.
The MP, who says he loves to dance, gave the impromptu display of his fancy footwork as he visited the closing day of a UK trade fair held at the shop, where shoppers ran into the store at opening time in the rush to snap up British produce.
The Conservative MP also mixed with Japan's fashion pack when he helped to launch a flagship store of Burberry, one of Britain's biggest clothing brands, in Tokyo.