We have to find out how this has actually been distributed, how it's been allocated and to take whatever action

Michael Meacher Environment Minister

The ash came from an incinerator in Edmonton, north London, and 50,000 tonnes of it was then supplied to construction projects by the company Ballast Phoenix, the programme reported.

David York, who runs the company, refused to tell Newsnight where the ash was used but said it had been agreed with the Environment Agency.

Newsnight said independent tests on a sample from Edmonton had indicated it contained 343ng of dioxin per kilogram.

That is 60 times the average amount found in soil, and 10 times the level that Mr Meacher estimated should be present in construction products.

Mr Meacher said the risk to human health must be checked and prosecutions brought where possible.

'Panic people'

Changes to the law might be needed, he added.

"We have to find out how this has actually been distributed, how it's been allocated and to take whatever action," the environment minister said.

But he questioned whether it would be right to reveal where the ash had been used.

"I am absolutely a supporter of total transparency and honesty, but I don't want to panic people unnecessarily and we have to make a proper judgement," Mr Meacher said.

Mislead parliament

He added that he had never intended to mislead parliament about the average levels of dioxins used in construction materials.

Dioxins caused health fears earlier in the year over fears that dioxins released from pyres of animals slaughtered in the foot-and-mouth outbreak could re-enter the food chain.

Newsnight revealed that, in one of Britain's worst pollution incidents, ash contaminated with dioxins was spread on tracks, paths and even allotments across Newcastle.

Very rarely do you get such huge levels of dioxin put straight into area where they can go straight into the food chain

Alan Watson Environment consultant

Throughout the 1990s Newcastle City Council delivered 2,000 tonnes of the ash to allotments across the city.

It contaminated the soil and dioxin levels rose to up to 300 times the usual amount.

Environment consultant Alan Watson told Newsnight it was one of the worst incidents of contamination in the world

"Very rarely do you get such huge levels of dioxin put straight into area where they can go straight into the food chain

"It's quite amazing that the authorities acted in this way.

"It was grossly irresponsible."