Campaigners today rounded on Boris Johnson's mayoral strategy for cleaning up London's air, claiming that the measures to tackle a key public health issue are not "fit for purpose".

Johnson has been under pressure to produce a credible package of measures to tackle London's poor air quality, which sees an estimated 4,267 Londoners dying prematurely each year because of long-term exposure to airborne pollution, according to figures released by city hall this year.

Johnson's third and final version of his air quality strategy, published today, outlines a number of measures – most for improving London's air quality, which is the worst in the UK and among the worst among European cities.

Johnson has set a target of having electric black cabs entering the fleet by 2020. The strategy coincides with a government announcement giving details of 4,000 new charging points for electric cars across the UK.

In the interim, Johnson has put deadlines in place to get the dirtiest cabs off London's roads by refusing to license any taxi over 15 years old from 2012 – a move which is expected to see 1,200 cabs scrapped from the fleet of about 22,000. But critics say the mayor has ditched a tougher target of refusing cabs over 10 years old by 2015, which was set out in his previous draft, and revised down the reduction of a key pollutant, PM10, from 33% to 31% as a result.

Road transport is responsible for around 80% of airborne pollution in central London – where air quality is worse – with black cabs contributing 20%. To encourage London cabbies to switch to low-emission alternatives, the mayor has coupled the publication of his strategy with the announcement of a £1m pot designed to offer a reduction in the purchase price for taxi drivers upgrading to low-emission vehicles, and encourage the taxi industry to develop an affordable, workable vehicle in time for the 2020 target, so that all new taxis entering the fleet in ten years' time can be "affordable zero-emission taxi capable of zero-emission operation".

In the interim, Johnson wants a 60% improvement in fuel economy by 2015 compared to 2010.

All new taxis entering the fleet by April 2012 will also have to meet the strictest emission vehicle standards, and eco driving training for those learning to be black-cab drivers will be made mandatory from January the same year - in effect a "greener knowledge".

Johnson also intends to revoke one of the first decisions he made after taking power in 2008, by returning to twice yearly MOT tests by 2013 at the latest, though these will be done "more simply and cost-effectively" at a local garage rather than via a few available inspection centres.

Johnson said he wanted the capital's taxi fleet to "match up to the highest environmental standards that a great city like ours deserves".

"From 2012 when the world heads to London, we will remove the oldest, dirtiest cabs from our streets," he said. "But we are also offering a juicy carrot, with the establishment of a fund to help speed up the introduction of electric black cabs. This forms part of a robust package of long-term measures to progressively clean up London's air."

The document seems little changed since its last draft. Measures in the wide-ranging strategy include using the planning system to make new developments "air quality neutral", and energy-efficiency programmes to reduce emissions from heating of homes and workplaces.

The mayor, who was widely criticised over his decision to delay the third phase of the emission zone in 2010 – for vans and minibuses – will introduce it in January 2012.

Johnson said the challenge of cleaning London's air is made more difficult because a significant amount (40%) of sources for both nitrogen dioxide (N02 ) and PM10s – comes from outside London.

He said further policies need to be taken at national government level if the European air quality standards are to be met.

The mayor stated that his strategy will meet the limit values for PM10s by 2011, but the clean air campaigner Simon Birkett said Johnson is "gambling" on the European commission agreeing to an extension on PM10s, while also admitting that he is "way off" meeting the European NO2 limits even if the government tries to apply for an extension to meet the standards, which were supposed to be met this year.

The government received a second and final written warning from Europe during the summer over the UK's longstanding breach of PM10 levels, due to levels in capital hotspots. The government faces the prospect of being taken to the European court of justice and face fines of up to £300m for being in breach, and is currently waiting the outcome by the European commission of a time-extension request to 2011.

Johnson stressed in the document that "everyone need to play their part" to improve air quality. Those who contribute most – buses, taxis and vans – should be expected to do more to provide the solution, but he urged central government to act. "We are calling on central government to implement new policies and provide funding for several initiatives to tackle this issue across the UK and therefore also London", said the mayor in the foreword of the 176-page strategy.

Darren Johnson, a Green party member of the London assembly, said:

"The London Mayor has dithered and delayed for two and a half years over a half-baked plan that doesn't do the job. He has even taken backward steps by delaying action against polluting white vans and getting rid of the Western Extension of the Congestion Charge [later this month]. He abolished six monthly inspections of black cabs, but now appears to be reinstating them from 2013."

Simon Birkett, director of Clean Air in London, said Johnson's

final strategy is still not 'fit for purpose', because by the mayor's own admission, annual mean concentrations of NO2 will still exceed the limit value by 2015 across 45% of London, including 65% of roads, unless further action is taken.

Birkett said: "The mayor has stumbled again after taking more backward steps on existing and proven air quality measures in the last two and a half years than he is committed to taking over the next 10 years."

"London's appalling air pollution is one of the biggest health problems facing the capital, leading to 4,300 premature deaths per year and countless thousands suffering ill health. It is therefore bitterly disappointing that the Mayor's strategy, after months in the making, still has a hole at its heart - the lack of effective measures to tackle pollution where it's worst in central London.

Mike Tuffrey, the Liberal Democrat London assembly environment spokesman said: "For almost two years I have called on the mayor to bring in a Berlin-style clear air zone, using the existing camera enforcement system to exclude the oldest and most polluting vehicles.

"The Mayor has failed to act on the evidence available and the best he is now offering is merely further talks with the boroughs, when instead decisive action is needed to protect the health of Londoners."