Millions of people given extortionate car parking charges outside train stations, NHS hospitals and supermarkets could be in line for a refund thanks to a new campaign.

Cambridge law graduate Michael Green, 22, is leading a legal battle to have the 'fines' overturned in what could be Britain's largest ever group litigation.

Private companies which run car parks outside some of Britain's biggest store chains - including Aldi, Lidl, B&Q and PC World - will often allow customers to park for free, but threaten a large fine for exceeding a certain time limit.

However, Mr Green believes that these fines are unlawful - and is trying to gather 100,000 cases together to go to the High Court and prove it.

Clamping down: A new campaign is challenging extortionate penalty charge notices handed out by private companies outside hospitals, shops and train stations - and could get tens of millions of people a refund

If he wins the landmark legal case, then that could open the door for 'at least 10million people' to claim their money back, and could put a stop to the fines for good.

He said: 'These "fines" are not official fines, but instead a matter of private law between two parties.

'When you park in a car park you are entering into a contract with the company which provides that parking space.

'If you breach a contract then under contract law that company is only allowed to make a claim for the amount they have actually lost.

'Because these spaces are free to park in, I would argue the amount is actually nothing.

'People take cases like this to the county court fairly regularly, and judges will often rule that the fines are unenforceable and the charge will be overturned.

'However, because of the way the county court system works, this will only settle the claim between one person and one company.

'By gathering the cases together, and taking it to the High Court, we can set a legal precedent.'

When a judge made a similar ruling over PPI, it opened the door to a flood of claims that has seen banks set aside tens of billions of pounds in repayments.

In the case of PPI, the ruling applied to 90 per cent of policies sold. Mr Green believes a new ruling could apply to 98 per cent of parking fines handed out by private companies.

Raking it in: Money collected from parking charge notices has almost doubled in the last four years alone

Growing problem: Each time a private company issues a fine, they request the driver's details from the DVLA. This data shows that, in 2014 alone, nearly 2.7million fines will be issued

An estimated 2.2million such fines were handed out last year alone, and that number is set to rise to 2.7million this year.

In total, it is estimated that these companies will collect around £127million in so-called 'fines' this year alone, almost double the £93million collected in 2010, and a 600 per cent increase on 2006, when just £21million was charged to motorists.

Mr Green added: 'In my opinion [the charges] are unlawful. It is remarkable that the parking firms are happy to send out so many "fines", despite their legality not being clear.

'To make an estimated £176m industry out of this, without having clear legal precedent to do so, is concerning.'

He is currently gathering interest on his Challenge the Fine website, before deciding whether to go ahead with the scheme.

If enough people sign up, he will attempt to gather 100,000 cases together before going to court.

He added: 'I think the law is pretty clear in this area, as evidenced by the conclusions of county court judges.

'But the law on how to organise such big cases is uncertain. We're going to be asking the judge some pretty novel questions so we might get some pretty novel answers.'

Among the companies using the potentially illegal practices is UKPC, which manages spaces outside NHS hospitals, along high streets, and beside major retailers such as M&S and Tesco.

Fighting back: Cambridge law graduate Michael Green is trying to gather 100,000 cases together to take to the High Court, which will make his case the biggest ever group litigation in UK history

Other companies include Parking Eye, which controls spaces outside many budget supermarkets including Asda, Lidl and Aldi, and Total Parking Services, which deals largely with shopping parks and stores such as B&Q and PC World.

The companies will issue ‘parking charge notices’ – named to replicate official council ‘penalty charge notices’ – but which do not have the same legal standing as official fines.

A growing number of people are refusing to pay, and because of this firms often follow the 'fines' with letters threatening court action, in the hope of browbeating people into submission.

Simply ignoring the legal summons is not enough, and will often end up with a person being issued with a CCJ - which could prevent them from taking out loans or getting mortgages in the future.

Mr Green, from Chatham in Kent, attended Clare College at Cambridge University, earning a Bachelors degree in 2013, and passing a Law Masters in 2014, receiving some of the top grades in his year.

Rather than begin training as a lawyer, he decided he wanted to start up a company giving everyday people greater access to the courts.

The idea to help fight parking fines came to him after a friend was hit with an excessive charge. In the future, he also hope to help fight rouge telephone callers.