A new law banning legal highs in the UK appears to be in crisis already after two major cases collapsed when judges agreed 'hippy crack' is not illegal.

Hundreds have been arrested and 50 dealers prosecuted and even jailed under the Psychoactive Substances Act since it came into force in May 2016.

Sellers of 'hippy crack' for recreational use were warned they faced up to seven years in prison under a Government crackdown on legal highs.

But now two trials involving the supply of 'hippy crack' at music festivals have ended abruptly after it was argued the drug is exempt from new legislation.

Even the Crown Prosecution Service's own top witness admitted that nitrous oxide is exempt under the act's 'current wording', which could lead to cases being axed, convictions quashed and people released from jail, experts have said. Ryan Egan and Kenan Buckley, who are in their twenties, were both arrested after carrying nitrous oxide into Glastonbury last year and charged with one count each of possession of a psychoactive substance with intent to supply.

But their defence QCs said that the laughing gas cannot be counted as a 'psychoactive substance' and Judge Paul Garlick greed and threw out the cases at ring the hearing at Taunton Crown Court. Southwark Crown Court where a defendant, who cannot be named for legal reasons, was accused of intending to supply nitrous oxide at a music festival in Derbyshire also collapsed.

Prosecutor Adrian Fleming admitted expert witness Professor Philip Cowen has 'the firm view that nitrous oxide, as the legislation is currently worded, is an exempt substance'.

Mr Fleming also said the CPS had told him the situation demanded 'a full review of the legislation and that will be carried out'. 

Drug charity Release says the cases prove the new law is 'fundamentally flawed' and has called for all cases to be reviewed.

Kirstie Douse, head of legal services, told BBC Radio 4’s Today programme: ‘It's something that we've warned about and this is now wasted resources.

‘These are two defendants who've had the confidence to challenge the law. But there’s plenty of people who've gone before that – I think around 50 people have been prosecuted for supplying nitrous oxide.

‘We think that those previous prosecutions and convictions need to be reviewed as well.

'I don’t know the reasons why those previous people were convicted – they may have pleaded guilty for fear of getting a custodial sentence; there may have been some kind of bargaining going on there.

‘I would certainly urge anyone who has been affected by specifically the nitrous oxide part of the legislation and other parts to contact their solicitors for further legal advice. Former deputy drugs tsar Mike Trace told BBC Radio 4’s Today programme: ‘If legal challenge succeeds against a carefully drafted act, then that's going to put the cat amongst the pigeons somewhat.

'Some of this was foreseen. When the act was passed last year, Release and many other charities were saying there were certain definitions in the act that would be open to challenge – this is just one of them.

‘To my knowledge this is the first successful challenge, but there may be more problems to come.

‘The Government over the last couple of years were right to try and think how they can get around this conundrum of trying to pass a law for each individual drug, or a schedule, so trying to make much broader definitions.

‘But it was obvious that if you come to those much broader definitions and say everything is controlled or prohibited, then there are going to be some very difficult definitional issues. 

'For drugs to be included under this act, the law has to prove they have a psychoactive effect on humans. And for the vast majority of drugs circulating out there, there's no research to show that, so there are going to be some challenges.’

In July last year, William Cook, 29, became one of the first people in Britain convicted under the new law. 

He was jailed for 42 months after admitting possessing nitrous oxide with intent to supply at the Electric Daisy Carnival in Milton Keynes. 

And just eight days ago a geography student became one of the first people to be sentenced under new laws for peddling bag full of potentially fatal 'hippy crack' at a festival.

Nicholas Chroussis was charged with possession of the legal but pyschoactive drug after being found with almost 250 nitrous oxide canisters, a dispenser and 250 balloons, police said.

When Sussex Police revealed text messages on his phone that showed that he was planning on selling the cannisters to other festival goers, he admitted the offence.