Medicinal cannabis was confiscated from a woman as she tried to bring the drug into the UK illegally for her daughter, who has severe epilepsy.
Emma Appleby and Teagan, nine, were stopped at Southend Airport after they flew from Amsterdam.
Mrs Appleby bought £4,000 of the THC oil capsules in the Netherlands after being refused a prescription in the UK.
But it was seized by the Border Force before the family, from Aylesham, Kent, was released from the airport.
It was illegal to bring the drug into the UK without a prescription, which doctors have been able to issue legally since 2018.
Doctors in the UK have refused to prescribe Teagan THC, a psychoactive compound found in cannabis. But Mrs Appleby believes the drug will help reduce her daughter's symptoms.
She bought a three-month supply of THC and Cannabidiol (CBD), using money raised through crowdfunding, at a pharmacy in The Hague.
"I'm absolutely gutted," she said after the drugs were seized. "They just took everything."
Speaking in the Netherlands on Friday, Mrs Appleby said her daughter had seizures "every single night, every single day and I don't know if she's going to wake up in the morning".
"This is our last resort. There's nothing else. We've tried all the medications at home," she explained.
"If there's a single, slight chance that this medication will help and save her I'm going to be here."
'Duty to enforce law'.
While it is legal in the UK for specialist doctors to prescribe THC, in general they will not because they say there is a lack of evidence that it's safe and effective.
The government says it has asked for new guidelines to be drawn up for doctors, and is encouraging further clinical research.
A government spokesman said: "The decision to prescribe cannabis-based products for medicinal use is a clinical decision for specialist hospital doctors, made with patients and their families, taking into account clinical guidance, which is based on the best international evidence.
"The Border Force has a duty to enforce the law and stop the unlawful import of controlled substances into the UK."