Millions of drivers face confusion about how they might be affected by a crackdown on older polluting vehicles, amotoring firm has claimed.
The RAC warned there was no easy way of finding out a vehicle's Euro emissions standard, which dictates the maximum permitted emissions under laboratory conditions.
It called on the Department for Transport to develop a dedicated website, as such a service was already available to car owners in a number of other European countries.
The Government has said local councils could bring in clean air zones with charges for the dirtiest vehicles based on Euro emissions standards.
An RAC survey of 2,200 drivers found that nearly two in five (38%) had not heard of the classification system, while 64% of those who had were unsure what category their own vehicle fitted into.
Information widely available online states that cars registered from September 2015 are Euro 6, which meet the strictest pollution tests.
But Euro 5 cars manufactured before June 2015 could still be sold up to September 2016, meaning some cars carrying a 65 number plate are Euro 6, and some are Euro 5.
RAC spokesman Rod Dennis said: "It is simply unacceptable that there is currently no easy-to-use or conclusive online look-up system available.
"This will no doubt leave drivers confused about whether or not they are likely to be impacted by the introduction of clean air zones."
He claimed the only source of detailed vehicle emissions data covering different manufacturers in the UK was the Vehicle Certification Agency's website.
But this is "a body many drivers will probably never have heard of", requires users to provide several pieces of information about their vehicle and features a "significant caveat" that people should also contact manufacturers, Mr Dennis said.
He called for the Driver and Vehicle Licensing Agency to create a website where people could simply enter a number plate to find out the Euro emissions standard of their car.
This must be made available before clean air zones were introduced, he said.
Last month a £10 toxicity T-Charge was introduced in central London for vehicles which do not meet the Euro 4 standard, generally those registered before 2006.
It covers the same area and operating times as the existing congestion charge zone in the centre of the capital, which runs on weekdays between 7am and 6pm.
Martin Tett, the Local Government Association's transport spokesman, said: "It's essential that government and manufacturers work together to make sure that vehicle emission standards are clear, up front and reflect the actual emissions released.
"This will help motorists play their part in making sure that our communities have cleaner air."
A Government spokeswoman said: "It's important that motorists understand whether their vehicle meets emissions standards and we are working with a number of parties including the DVLA to make sure this information is easily available."
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