Heathrow has massively underestimated the cost of soundproofing homes affected by a third runway, say campaigners.

Bosses at the airport have set aside £700m to provide noise insulation for 160,000 homes should it get the go-ahead for an extra runway.

But anti-Heathrow expansion protest group HACAN says its research shows the true cost would be £1.8bn.

The group calculated the figure following consultation with UK Soundproofing Ltd of West Sussex and Tudor Windows of London, which it said showed the average semi-detached house would cost around £11,800 to fully insulate against noise.

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HACAN chairman John Stewart said: "Heathrow is offering to insulate many more properties than it does today. Our supporters welcome this but only if their homes will be fully insulated. The cost of doing that would run into billions.

"Admittedly, our estimates are based on costs for an individual home and there would be discounts if an order was placed, but it will still cost substantially more than the £700m originally put forward."

Heathrow Airport has promised to quadruple the number of homes eligible for noise insulation should it get a third runway, as it nearly tripled the amount of compensation available.

A Heathrow spokesperson said: "Heathrow's noise insulation scheme is privately funded and has been created following several years of public meetings in our local boroughs and after receiving feedback from more than 2,000 residents.

"We recognise that our expansion plans need to treat people fairly, and so we extended our noise insulation offer to all homes based on the 55 decibel noise contour, the preferred measure of noise used by the European Union and the Mayor of London.

"Heathrow's plans for expansion can be delivered on time and on budget. In fact, Heathrow already has a strong track record in this regard, having invested £11bn over the past decade on transforming the airport building Terminal 2 and Terminal 5."