Two hemp farmers say they have been left devastated after they were forced to destroy 40 acres of the crop – the end product of which can be bought legally in high street shops.

Patrick Gillett and Ali Silk said they had to cut down their crop because the Home Office said they were no longer allowed to harvest it for cannabis oil, or CBD.

As they surveyed the Oxfordshire fields where their combine harvester tore up the hemp, the pair remained perplexed over the order to cease production at their cooperative farm.

Gillett and Silk are so incensed over what they believe was an unjust order from the Home Office that they have launched a national campaign to have the policy reversed.

“For three years we operated openly and always kept the Home Office informed over what we were doing,” Gillett said as he lifted one of the remaining stalks scattered around the field.

“It was devastating to have to rip the entire crop up just because the Home Office changed its guidelines. In fact, one of their guidelines is that any cannabis oil extracted from the hemp plant only contains 0.2% of THC [tetrahydrocannabinol, the substance that produces a psychoactive high], which we also stuck by and indeed were upfront with the Home Office. We kept them informed every step of the way when we started this business in 2015.

“It seems like we are being punished for being upfront and honest about what we were doing, when we took the decision to extract cannabis oil solely for wellbeing purposes. Once we got started in 2016 after learning about CBD oil and how it was doing in America … we never hid it.”

Samples of Hempen products. Photograph: Martin Godwin/The Guardian

Silk, who gave up a job in the City of London this year to pursue her dream of a more eco-friendly, organic lifestyle, said: “We are being banned from producing something organically, something which is also good for the environment by its carbon capture, that is available in shops on any British high street. You can go into health and wellbeing stores and buy CBD oil over the counter yet we can’t produce it here in these fields in England because the Home Office treats hemp like narcotics or firearms.”

Silk estimated that their company, Hempen, would lose about £200,000 as a result of the destroyed crop and while their overall projections for sales over the next few years were as high as £2.4m, she said it would be foreign hemp growers who would reap the benefits.

“Patrick and I worked it out that about £480,000 of our profits over the next few years would be taken away in tax. So that is a massive loss in tax revenue plus the foreign producers we can actually buy from, Swiss hemp farmers, will be the ones to benefit from the Home Office ban,” she said.

As the sun beat down on a slashed and stubbled field, Gillett said: “This is all very strange. No one in the Home Office up until the end of last year ever said to us: ‘Stop what you are doing, this is illegal.’ They allowed us to get started and then after a perfect year – wet spring and hot summer – we had a bumper crop which they made us destroy last Tuesday. It doesn’t make sense.”

A Home Office spokesperson said it did not routinely comment on individual licences. The tenant farmer had held a hemp grower’s licence. However, it is understood that his application to renew the licence was recently rejected by the Home Office.

Silk and Gillett said they sought cooperation from the Home Office and wanted to be fully licensed “because any CBD oil we would produce would have been well within the legal guidelines of 0.2% THC”.

Gillett said Hempen had been in touch with farmers from Yorkshire to the Channel Islands in an attempt to build a national campaign to overturn the Home Office ruling.

“We need to persuade the government to take these decisions away from a department that deals with illegal drugs and guns, and put it in the hands of somewhere like the Department for Environment, Food and Rural Affairs. We need to get them to see the benefits of CBD and hemp in general rather than look upon it as a criminal matter.”

• This article was amended on 7 August 2019 because an earlier version wrongly suggested that Hempen had never held a Home Office licence to grow cannabis. It is understood that the tenant farmer had held a licence for three years, but the Home Office recently rejected his application for the licence to be renewed.