Air pollution in London has hit its highest level since 2003 according to official figures.
Under European Union air quality laws daily pollution levels must not be above the legal limit on more than 35 days in a calendar year.
But air pollution levels in London have already exceeded EU daily limits 36 times this year.
It is the first time that the annual limit has been exceeded by this date since 2003 when there were 50 breaches.
On Wednesday, Marylebone Road in central London exceeded EU limits for the 36th day this year.
Tiny sooty particles
The station there records air pollution from tiny sooty particles, known as PM10, which are mainly caused by traffic.
The government has issued a "smog alert" for the Easter weekend for PM10s, which can affect people's health.
Simon Birkett, director of Clean Air in London, said: "We need mayor Johnson and the government to tackle an invisible public health crisis with as many premature deaths attributable to air pollution in London in 2008 as we thought occurred during the Great Smog of 1952."
The European Commission has already given the UK two written warnings and the next stage in the legal process will be referral to the European Court of Justice.
The Commission gave the mayor and UK Government until June to come up with a "London action plan" to ensure compliance with the air pollution limits.
The UK will not face fines for the breach, as the EU has given Britain an extension until June before it has to start meeting the standards in the capital.
James Grugeon, chief executive of the charity Environmental Protection UK, said the breaches showed official assurances that pollution was under control in the capital were not true.
He said: "The mayor's air quality strategy, even with a new action plan set to start in June, is not up to the job of tackling the high levels of pollution already recorded this year."
A statement from London mayor Boris Johnson said that until 11 June, the daily limit of PM10 in London was 50% higher as a result of the UK's extension from the EU.
The statement says that as of 19 April, the daily limit value has been exceeded at Marylebone Road just six times this year, meaning that the annual legal limits have not been breached.
A spokesperson for the mayor said: "The mayor is already taking action to improve London's air quality with cleaner buses, tougher standards for the Low Emission Zone and the first ever age limit for taxis.
"Since the beginning of 2011, we estimate that more than 75% of air pollution episodes have occurred when pollutants have been blown in from Europe."
But Simon Birkett said the extension was only valid when the air pollution action plan is implemented and until then the lower limits apply.