The sheer scale of the challenge facing the government as it tries to meet its own targets to cut carbon emissions from public buildings was underlined today after it was revealed that many of the UK's most iconic buildings are dogged by extremely poor energy efficiency.

Under new legislation taking effect from today, about 18,000 public buildings larger than 1,000 square metres will have to exhibit Display Energy Certificates (DEC) detailing how energy efficient the building is on sliding A-G scale.

But in an embarrassing development for the government, of the 3,200 buildings assessed so far a quarter have been awarded an F or G, with many high profile buildings, including the Palace of Westminster, the Bank of England and the Natural History Museum receiving the bottom ranking.

While the poor performance of many older buildings may have been expected, several newer buildings also performed poorly with London's City Hall being awarded an E rating, despite being opened only six years ago, and the Imperial War Museum North, which was also opened in 2002, scoring a G.

The findings will add to criticism that the government is not practicing what it preaches with regards to green building improvements and highlights the scale of the challenge it faces if it is to meet its target of cutting emissions from the central government estate by 30 per cent by 2020.

A representative for the Office of Government Commerce (OGC), which is responsible for the management of the public sector estate, insisted that the government remained committed to meeting its various environmental targets on energy, water and waste across its buildings.

He added that the government had recently launched a new "plan of action" to drive improvements across its estate and that the introduction of energy certificates would offer further incentives to departments to improve their performance.

John Alker of the Green Building Council said that it was unsurprising that so many government buildings had performed poorly.

"The rating is based on a ratio of actual energy used against an optimally designed building and the benchmark for average performance is around band D," he said. "If you look at buildings like the Palace of Westminster it is no surprise that it comes in below the average – but what this does serve to do is highlight the scale of the challenge."

He added that the glut of band G awards was likely to have the intended effect and galvanise more building managers to try and improve their energy efficiency. "A lot of the improvements that can be made are just about managing the building better," he explained. "So if this does get more directors engaging with their facilities managers and asking why we have got a band G rating then it is working."

Housing Minister Iain Wright MP similarly argued that the certificates would have a positive impact. "Display Energy Certificates are a valuable tool in the fight against climate change, with this ambitious programme showing how building performance can be improved, saving not only carbon, but public money," he said.