Heathrow remains vital to the UK economy, Ruth Kelly said
London mayor Boris Johnson's idea of building a new airport on the Thames estuary is "irresponsible", Ruth Kelly has told the Labour conference.
The transport secretary said the idea, aimed at easing congestion at Heathrow, was also "opportunistic" and would prove "environmentally disastrous".
The mayor's office said it was right to examine alternatives and a feasibility study was the correct way forward.
Ministers are set to decide on a third runway at Heathrow later this year.
The government supports the idea but the Conservatives and the Liberal Democrats say Heathrow must be made to work better, rather than being further expanded.
The Tory mayor has said policymakers must look at alternatives to the "endless expansion" of Heathrow, saying a new airport located on an artificial island off the Isle of Sheppey in Kent was definitely worth "looking at".
But Ms Kelly - in her last major speech as transport secretary after announcing her intended resignation - derided the idea.
"Boris may find his fantasy island airport a useful device to distract from the Tories divisions on Heathrow," she said.
"I call it politically opportunistic, economically irresponsible and environmentally disastrous."
London needed a "hub" airport, offering connections all over the world, to support the country's economy, Ms Kelly added.
Heathrow handles more than 500,000 flights a year but its owner says the airport is "jam-packed" and needs a new runway to be able to compete with other major airports in Europe.
But the prospect of an additional runway is hugely controversial and is fiercely opposed by local residents and environmental groups who fear the impact on air quality and noise in the area.
But there are also serious question marks about the sustainability of an airport on the Thames estuary.
Conservation groups say the impact on wildlife could be "damaging".