This case is about the right to breathe clean air and could have a huge impact far beyond the UK's borders - it could force governments across the EU to take action.
ClientEarth and European Commission lawyers have told EU judges that under the UK Government's pollution plans, air quality will not meet nitrogen dioxide limits in three major urban areas until after 2030.
The UK tried to suppress this information using rules on legal privilege -but failed because the information it sought to conceal was inadvertently published on DEFRA's website last night.
The information had previously been disclosed by the UK in legally privileged correspondence responding to a separate legal action brought by the Commission in February this year, but was not made public.
The 'secret' report shows that even after 2030, three major urban zones will be breaching the air quality targets: Greater London, the West Midlands (Birmingham) and West Yorkshire (Leeds). Portsmouth and Southampton are expected to meet the target by 2030.
Until now, the government has maintained it would meet nitrogen dioxide limits by 2025 in London and by 2020 in 15 other zones.
"It is quite extraordinary - we did not expect that", said a ClientEarth spokesman. "The UK argued that the information should be suppressed, but our barrister pointed out that it was already in the public domain following its publication, and that was really the end of the argument."
The new figures are based on recalculated emission factors for nitrogen oxides from road traffic, using "more accurate assumptions on the performance of modern diesel vehicles and older petrol cars". They also assume that no new measures to tackle NO2 beyond those included in the plans published in 2011 are taken.
The poor result, says the report, is "largely due to the failure of the European vehicle emission standards for diesel cars to deliver the expected emission reductions of NOx."
ClientEarth's case was referred to the Court of Justice of EU by the UK Supreme Court. Commission lawyers described the case as: "A matter of life and death" and said this would be "perhaps the longest running infringement of EU law in history."
Nitrogen dioxide is a toxic gas produced mainly by diesel vehicles. Every year, 29,000 people die early in the UK as a result of air pollution - more than traffic accidents and passive smoking combined.
Alan Andrews, ClientEarth lawyer, said: "It's bad enough that the government has no intention of complying with these limits in the foreseeable future. It's even worse that they're trying to hide behind legal procedural rules to keep this quiet.
"We have a right to breathe clean air and the right to know when the government is failing to protect us. Another five years of delay means thousands more people will die or be made seriously ill. The UK needs to act now to get deadly diesel vehicles out of our towns and cities."
Supreme Court: Government in breach of its legal duty
Last year, the UK Supreme Court declared the UK Government is breaching its legal duty to achieve limits for nitrogen dioxide. It then asked the CJEU to rule on what remedial action it can compel the government to take.
The CJEU's judgment is expected before the end of 2014. It will be binding on the UK courts and the national courts in all 28 EU member states. The case will then return to the UK Supreme Court in early 2015 for a final ruling.
"This case is about the right to breathe clean air and could have a huge impact far beyond the UK's borders", said Andrews. "It could force governments across the EU to take action."
The 'secret' government report: Updated projections for Nitrogen Dioxide (NO2) compliance.