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By Tom Richardson
BBC Newsbeat
A festival boss has formally threatened legal action against the government over drug checking at festivals.
Parklife founder Sacha Lord has written to Home Secretary Suella Braverman urging her to allow on-site testing in "pop-up labs" to go ahead.
He says festivals had been doing this for at least 10 years until last month, when the government told him a licence would be needed for the first time.
The government says a licence has always been required to test drugs.
Drug-checking is where illegal substances are tested and notifications put out if any are found to be dangerous.
Supporters say these warnings save lives and also give medical teams a better idea of how to treat anyone who becomes seriously ill after taking drugs.
UK festivals have most recently employed "back-of-house testing", which uses samples of confiscated or surrendered drugs.
Larger festivals like Glastonbury, Reading and Leeds have told BBC Newsbeat they've used private companies to do this.
Independent festivals have tended to use charity The Loop, whose volunteers have been able to test at events without a licence due to agreements with local police and councils.
Mr Lord's letter says the Home Office was "well aware" of these arrangements and former ministers have publicly said the department wouldn't intervene to stop them.
He argues that the department made a "flawed decision" when it insisted on licences being obtained this year.
The letter says festival organisers had a legitimate expectation they would be able to test as normal this year and weren't properly consulted beforehand.
It says notification of the licence requirement - two days before Parklife in Manchester began - came too late for it to be possible to obtain one in time.
The letter, co-signed by trade body the Night Time Industries Association, demands the government allow testing without a licence to go ahead as before, or take steps so organisations have enough time to comply with the licence requirement.
They've given the government a deadline of 7 July to provide a meaningful response, and say they'll begin legal action if this isn't met.
Those behind the letter say they'll apply for a judicial review and ask a judge to examine the Home Office's decision.
The Home Office has been approached for comment on the legal letter.
The department previously told Newsbeat: "Our position hasn't changed. Drug testing providers must have a licence to test for controlled drugs, including at festivals.
"We have consistently made this condition clear, and law enforcement have always had a responsibility to uphold this legal requirement.
"We continue to keep an open dialogue with any potential applicants. Festivals aiming to test drugs off their site this summer must work with the police and a Home Office licensed drug testing provider."