Mayor of London Boris Johnson fired up the Conservative party conference yesterday with a rousing call to arms for Britain's 2015 election and a challenge to Russian President Vladimir Putin on cucumbers.

But the blonde-haired mayor took pains to stress his loyalty to Conservative Prime Minister David Cameron with frequent references to the good work of "Dave" and his ministerial colleagues.

The mayor was dismissive of recent reported comments by a Russian official that Britain was just a "small island" with no international influence.

"I don't want to risk polonium in my sushi by bandying statistics with the Kremlin," he said, a reference to the radioactive poisoning of ex-Russian agent Alexander Litvinenko in London in 2006,

But Johnson argued that London was home to so many rich Russians because of its numerous benefits, including green spaces that help it produce two million cucumbers a year.

"Eat your heart out, Vladimir Putin," he said, before adding: "It is partly due to our staggering cucumber yields, comrades, that London now contributes almost 25 per cent of UK GDP."