Drivers in London with vehicles fuelled by diesel will be charged an additional fee similar to the existing £11.50 congestion charge, if proposals by the mayor Boris Johnson are accepted.

The plans are aimed at reducing air pollution, which in the capital has reached levels far above legal safety limits, leading to the prospect of large fines from the European Union.

Johnson's extra charge is part of previously-aired plans to charge for diesel vehicles in his mooted "ultra low emissions zone", which would cover some of the most polluted areas. He is scheduled to make a speech tonight setting out his plans in more detail, but the proposals must go out for consultation before they can be accepted.

Johnson is also lobbying central government to raise taxes on diesel fuel, which is taxed at the same rate as petrol despite the fact that it is much more polluting. As diesel cars burn less fuel over a similar distance than petrol ones, the equal rates of tax effectively favour diesel.

Simon Moore, senior research fellow at the thinktank Policy Exchange, said: "For too long people have been encouraged to buy diesel vehicles, which while slightly better for the climate are far worse for local air quality. The mayor's proposal to charge diesel drivers extra to come to places where pollution is worst should help end this dire health problem. It gives a clear sign that polluters should pay."

Murad Qureshi, environment spokesman for the London Assembly Labour Group Environment, call Johnson's proposal half-baked. "Instead of banning diesel vehicles completely from an ultra low emissions zone, the mayor is simply fining them £10 for the privilege. And by not implementing this until 2020, Boris is kicking the problem into the long grass.

"By that time, Boris will have left City Hall four years previously, and around 51,000 Londoners will have died prematurely since he took office in 2008. London needs bold action now. The mayor should bring forward these proposals from 2020 to an earlier date."

The Green party London Assembly member, Jenny Jones, said: "The mayor is right to target diesel vehicles as a major health hazard but we need action on them immediately, not 2020. Also, the ultra low emissions zone in central London is too small to have any significant impact on the majority of diesel vehicles in London, which is why we need the mayor to allow all the inner London boroughs to join the scheme if they want to."

Diesel vehicles produce high levels of fine particles, like soot, that can cause respiratory problems in children and susceptible adults.

The number of diesel vehicles on UK roads has been steadily rising – by 2012, half of new vehicles sold were diesel – in part because of the favourable tax situation and partly because they are cheaper to run.

Diesel cars were once thought by some to be more environmentally-friendly than petrol models, because they produce less carbon dioxide, but concerns have been raised in recent years over their air pollution impact. An estimated 30,000 premature deaths a year are caused by air pollution in the UK, according to Public Health England.

In April, days of severe smog in the capital led to older people, small children and people with asthma being advised not to leave home.