Managers at Didcot power station revealed today they lost £500,000 when Greenpeace protesters forced them to reduce production.

Thirty Greenpeace protesters broke into the site early yesterday. Fifteen protesters chained themselves to a conveyor belt in a bid to halt coal-fired production, while a further 15 scaled the 650ft chimney.

Two then abseiled down the stack to paint "Blair's Legacy" on the side and the Prime Minister was questioned on the issue when he visited businesses in the area.

The protesters said this morning they had spent a comfortable night at the station, had enjoyed their breakfast, and pledged to remain for another night.

Kelly Brown, a spokesman for npower, confirmed that the protest cost npower an estimated £500,000 yesterday because the plant was forced to halve its normal output once the coal conveyor belt was out of action.

She said a secondary conveyor belt on another part of the site was now being used and that the coal silos have been filled.

She said: "We still won't be at full capacity today but we are in a better position than we were yesterday."

Jen Corby, a fellow spokesman for npower, added: "The police are also still on site and we have been working closely with them throughout the night to resolve the situation. "We remain very concerned for the safety of those onsite while this continues.

"We are using an alternative route to transfer the coal into the boiler and expect to be burning coal along with gas during the day, especially at the peak demand time this evening.

"We have weeks and months worth of stocks onsite and rail deliveries of coal to the site can continue as normal."

Kevin Akhurst, managing director of generation and renewables at npower, said: "Regarding Greenpeace's point of view, we fully support clean energy and we're at the forefront of developing it.

"We're the leading operator and generator of wind power in the UK with 16 on and offshore wind farms, as well as many hydro power stations and a fleet of highly efficient combined heat and power plant.

"We're also a leading user of carbon neutral "biomass" and we're developing plans which could lead to investment of hundreds of millions in groundbreaking clean coal' power technology.

"Of our three coal stations, Didcot A and Tilbury are already scheduled to close over the next five to 10 years, as are many coal stations in the UK, but they can't just be switched off overnight.

"Some coal is needed to maintain secure and affordable energy, and for now stations like Didcot meet that need.

"We've already reduced emissions in recent years at Didcot by improving its efficiency and most recently increasing the biomass fuel burned there. This could potentially save up to 700,000 tonnes of CO2 per year."

Police spokesman Kate Smith said officers were still on site negotiating with protesters and added that "safety considerations" were paramount.

"Both locations are high enough to kill someone if they fell," she added.

Ms Smith said potential charges including aggravated trespass and criminal damage for painting the chimney.