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Gordon Brown's pledge to create a string of carbon-neutral “eco-towns” will prove unachievable unless the government buys carbon offsets, the designers of the scheme have warned.

The offsets, potentially costing millions of pounds a year, will be needed because eco-towns will be net generators of greenhouse gases, even though ministers have promised they will be zero-carbon. The warning comes from the Town and Country Planning Association (TCPA), the government’s main adviser on the project.

David Lock, chairman of the association, said: “There has never been a true zero-carbon settlement and the eco-towns will not achieve it either. They will generate much less greenhouse gas than normal but to call it zero-carbon is slack language.”

Lock, a strong supporter of the eco-town concept who is advising developers on four of the proposed settlements, said the zero-carbon claim was raising expectations too far. “These phrases are like slogans on a T-shirt but if you ask civil servants and ministers what they mean, they cannot define it,” he said.

The warning comes as the Department of Communities and Local Government (DCLG) prepares to announce the sites of the first settlements, possibly as early as this week.

The eco-towns plan was announced by Brown after he became prime minister last summer. He asked developers to submit proposals for new settlements of up to 20,000 homes.

It was part of an attempt to regain control of the environmental agenda from David Cameron and achieve the government target of 3m new homes by 2020.

Yvette Cooper, then housing minister, told last September’s Labour conference that her aim was to develop “eco-towns with zero-carbon offices, zero-carbon schools and zero-carbon pubs”.

About 70 applications have been received, with Caroline Flint, the current housing minister, set to announce soon a short-list of 15, with 10 eventually being built. “Climate change is perhaps the most pressing issue facing not only this country but the entire world,” Flint said. “By 2016 all new houses will have to be zero carbon while all new nondomestic buildings will be zero carbon from 2019.”

Bill Dunster, an architect renowned for leading the way on green building design said he did not accept zero-carbon developments were impossible. Dunster's own BedZed development on a disused sewage works in Beddington, south London, is seen as an exemplar of the kind of projects needed if ecotowns are to become viable. He said: "The Code for Sustainable Homes sets out just how new housing can be made carbon neutral. It is technically very feasible. The real obstacle is the construction companies who are very conservative and resist anything they fear might reduce their profits."

The confusion over whether the towns can be zero carbon is the latest row to hit the plan. It has already provoked widespread opposition from residents near the sites of the proposed developments.

Felix Dennis, the multi-millionaire publisher, is fighting plans for an eco-town near his home in Warwickshire. Tim Henman, the former world No 4 tennis player and his parents are opposing a project in their home village in Oxfordshire.

The DCLG is understood to be holding urgent consultations over what zero carbon should mean and the role of offsetting. Opponents are angry that the building of supposedly environmentally friendly housing estates will concrete over countryside, some of it rich in wildlife, which they previously believed inviolable.

Among the protesters is Tessa Robertson, 48, whose 500-acre farm near Micheldever, Hampshire, faces destruction. Robertson has worked for Greenpeace and the WWF but said the new towns were “greenwash for developers”. “It cannot be green to destroy prime farmland and ruin our wonderful hills,” she added.

Douglas Paterson, 46, a neighbouring farmer, said the 350 acres he would lose to the development produced enough wheat for 1m loaves of bread plus feed for 200 dairy cows each year. “The world is short of food and this is prime farmland they want to build on,” he added.

Kate Gordon, a senior campaigner on planning at the Campaign for the Protection of Rural England, said that the ecotown concept was a good one but would only remain credible if the principle of sustainability were rigorously applied. She said: "Offsetting is not the way forward since if fails to actually address the problem - our dependency on fossil fuels. This is a hugely disappointing admission on the part of the TCPA."

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