Public buildings in England and Wales are pumping out 11m tonnes of carbon dioxide a year, more than Kenya's entire carbon footprint, the Guardian can reveal.

Unpublished findings of an energy efficiency audit of 18,000 buildings including ministerial offices, police stations, museums and art galleries reveal that the 9,000 buildings audited so far produce 5.6m tonnes of CO2, with one in six receiving the lowest possible energy efficiency rating.

The carbon dioxide they produce is the equivalent of all the greenhouse gas emissions saved by the UK's wind power industry.

Ignorance among officials, inefficient equipment and poor energy management have been cited as reasons for the result, which was described as "lamentable" by environmental campaigners. It comes despite ministerial pledges to slash the carbon footprint of government offices by 30% over the next 12 years compared with 1999-2000 levels.

Officials expect the carbon footprint to double when the audit is completed. Almost half of those tested so far have received an energy efficiency rating of E, F or G, the lowest possible and the equivalent of a gas guzzling car.

Embarrassingly for Ed Miliband, the energy and climate change secretary, his department's head office in Whitehall Place is one of the worst offenders. It pumps out 1,336 tonnes of CO2 a year and received a G rating. The Houses of Parliament and the Bank of England together consumed enough electricity and gas to emit 21,356 tonnes of CO2 a year, the equivalent of more than 14,000 people flying from London to New York.

Almost 70% of public offices had a larger carbon footprint than a typical office, as defined by the government, and while only 55 of the 8,849 buildings examined so far received an A ranking, 1,514 scored a G. The data was released following a request under the Freedom of Information Act.

In response, environmental campaigners and opposition parties called on the government to invest in an urgent programme of refurbishment to reduce the carbon footprint of the public estate, and cut energy bills for the public sector which currently add up to around £4bn a year.

"This confirms that the leadership society needs from government on reducing carbon emissions from buildings isn't there," said Tony Juniper, an independent sustainability campaigner and former director of Friends of the Earth. "For the UK to have any chance of meeting the 80% reduction in carbon emissions by 2050, which is now enshrined in law, there has to be radical change in this area."

Earlier this year, the government's own architecture adviser, the Commission for Architecture and the Built Environment, said the majority of government departments were "failing to make their new buildings and refurbishments sustainable" and that those operating them had little idea how to improve their efficiency.

"These figures show there is a desperate need to sort out the public building stock," said Steve Webb, spokesman on energy and climate change for the Lib Dems. "If business and householders see the public sector is not taking energy efficiency seriously, they will wonder why they should do so themselves. Instead of making a £12bn VAT cut, the government should have spent some of that money making public buildings energy efficient, saving money and carbon in the long term."

Buildings consume close to half of the electricity and heat produced by the power sector, according to Sir Nicholas Stern's review of the economics of climate change. On current trends, Stern predicted CO2 emissions from buildings will rise 140% by 2050.

John Alker, public affairs manager of the UK Green Building Council, said: "Many of our public sector offices, schools and hospitals are the building equivalent of gas-guzzling cars.

The government has set a target to cut CO2 by 30% from its own buildings by 2020. Frankly, they should do more because their track record leaves a lot to be desired. We need a comprehensive programme of green refurbishment in the public sector, which is responsible for about a third of all non-domestic buildings."

But plans to "green" the Palace of Westminster as an example to the rest of the public sector, have been scaled back, the Guardian has learned. Designs for wind and tidal turbines and solar panels to produce electricity are now unlikely to come to fruition after calculations that the investment needed would not result in quick-enough savings on energy bills.

A spokesman for the Office of Government Commerce, which has responsibility for the energy performance of public buildings, said it had set up a centre of expertise to help the public sector improve energy efficiency and meet government targets. "A comprehensive delivery plan has been produced detailing departmental activities to achieve the targets for sustainable operations across Whitehall, and real progress continues to be made," he said.