The supreme court today ordered the UK government to set out new plans to clean up Britain's toxic air.
The ruling means the next government must spell out new plans to reduce toxic levels of pollutants in the air by the end of the year.
The verdict is a huge victory in a five-year legal battle by clean air campaigners.
ClientEarth Lawyer Alan Andrews, who brought the action, said: "Air pollution kills tens of thousands of people in this country every year. We brought our case because we have a right to breathe clean air and today the supreme court has upheld that right.
"This ruling will benefit everyone's health but particularly children, older people and those with existing health conditions like asthma and heart and lung disease.
"The next government, regardless of the political party or parties which take power, is now legally bound to take urgent action on this public health crisis. Before next week’s election all political parties need to make a clear commitment to policies which will deliver clean air and protect our health.”
Until now, the government had hoped to delay action to tackle air pollution.
Last year ministers admitted they did not plan to meet legal limits on nitrogen dioxide (NO2) linked to diesel engines until at least 2030, more than 20 years later than originally planned.
High levels of air pollution are known to exacerbate conditions such as asthma and can cause breathing difficulties in otherwise healthy people.
There have been a series of serious smog incidents in London over the past year. However, London mayor Boris Johnson has so far been reluctant to take serious steps to reduce pollution in the capital.
Last year he dismissed air quality during the so-called 'Saharan smog' episode as "perfectly fine". Earlier this year he also suggested that air quality in London was so good that children should be "bussed in" from the countryside to breathe it in.
"The air quality in London, you can go outside and breathe in great gulfs of virtually alpine air," he told the think tank the Capital City Foundation in February.
"There was a recent bad air day - this is absolutely true - where the air quality in Norfolk was inferior to the air quality in London.
"The day is not far my friends when the children of Norfolk will be bussed in order to inhale our superior air."
Today's ruling comes as a new poll finds that most Londoners do not think politicians are doing enough to tackle air pollution.
The YouGov poll for the Evening Standard found that 69% thought political parties were either not doing enough or doing nothing at all, to tackle the problem.
The court's verdict means the next government and mayor will have to take the issue far more seriously.
It also throws into doubt the viability of several planned infrastructure schemes. Proposals to build new road crossings in East London and new runways in SE England are now likely to be under greater threat of legal challenge.
Reacting to today's verdict, Caroline Russell, Green Party local transport spokesperson, said:
"Finally, after a four year court case, the serious health threat posed by air pollution has been recognised by the courts. The government is now forced to take action to clean up our air and protect our health by meeting EU nitrogen dioxide limits. It is scandalous that they have delayed so long knowing the very serious public health impact of their failure to act.
"The speed of the judgement highlights the urgency of the issue. The seven month timescale for Defra to produce a plan to cut emissions must now concentrate minds and lead to immediate and widespread measures to clean up our air.
"This judgement is binding on whoever wins the election on Thursday. It must be a priority to reduce these unnecessary deaths, protect the health of children growing up in towns and cities and ensure all those people with underlying heart and lung problems are able to breathe confident that the air is clean."
Liberal Democrat chair of the London Assembly environment committee Stephen Knight said:
"Without more decisive action a child born today might be able to vote before they gain the right to breathe clean air.
"This ruling challenges the complacency that has for too long been shown by Whitehall - and of course Boris Johnson at City Hall - in tackling air pollution."
The government today insisted they were already working on plans to tackle the problem.
"Air quality has improved significantly in recent years and as this judgement recognises, work is already underway on revised plans (since February 2014) to meet EU targets on NO2 as soon as possible," A DEFRA spokesperson said.
"It has always been the government’s position to submit these plans before the end of this year. Meeting NO2 limits is a common challenge across Europe with 17 member states exceeding limits."