Teenage kicks for Kirani James as he sweeps to Grenada's first gold

On the first day of next month Kirani James celebrates his 20th birthday and will do so as a world and now Olympic champion. The teenager from Grenada last night comfortably won the 400m to complete a scintillating double.

It was a stunning performance from James, who won with ease from another 19-year-old Luguelin Santos of the Dominican Republic, finishing more than half a second clear. Bronze went to the Trinidadian Lalonde Gordon, a relative veteran at 23. This was a young man's race.

James, an easy, fluid runner clad in his green and red suit, crossed in 43.94sec – it is the fastest time ever run on British soil, 0.04sec quicker than the great Michael Johnson managed, although the latest pretender to Johnson's crown still has some way to go to challenge his world record of 43.18sec.

It was the best received victory of the night – James has quickly become a favourite with the Stratford crowd having swapped name tags with Oscar Pistorius following his victory in the semi-final.

A year ago in Daegu, James became the youngest winner of the world 400m crown and a year on he has earned the ultimate accolade. It never looked like ending any other way.

He has long been marked out for stardom, even in a country that has no great Olympic history – his was the country's first medal at the Olympic Games.

Three years ago, after a series of outstanding runs, including winning the world youth championships, there was a scramble among a number of US universities to provide his education, and acquire his athletic skills for their track and field teams.

He rejected approaches from Florida and Arizona before deciding on Alabama. James has continued to flourish in his new surroundings. Last year he ran in London for the first time and gave the capital a taste of what was to come with a personal best.

Last night was another new low for James, who shaved 0.42sec off his previous best in a sumptuous lesson in peaking at exactly the right moment. A personal best had secured him the world championship too.

There was no Belgium fairytale for the Borlée brothers. There was almost no separating them either as they finished fifth and sixth, 0.02 apart. The identical twins had bracketed the field, starting in lane one and eight but failed to match their season's bests, both of which would have won them a medal.