Kenyan khat farmers have threatened violent action against the British government following its decision to ban the stimulant.FG Machuma, who says he represents the Meru tribe, told Sky News the decision to make the plant a class C drug was a "declaration of war".
He added: "If they don't listen to us ... they have a military base in Nanyuki ... and they will have to leave.
"If they don't leave peacefully then we will take arms and deal with them in Kenya."
In the UK, khat is popular among some members of Somali and Yemeni communities.
The latest figures from 2011-2012 put the plant's UK value at £13.8m.
Users chew the leaves then swallow the juice, which contains an ingredient similar to amphetamine.
After a few hours, users become talkative and experience feelings of alertness, euphoria and excitement.
But symptoms can include depression, lack of concentration and psychosis.
The majority of British trade comes from the town of Meru in Kenya.
It provides a source of income for around 500,000 farmers, who say the ban threatens their livelihood.
Home Secretary Theresa May defied the Government's Advisory Council on the Misuse of Drugs to push through the ban, which was approved by the House of Lords on May 12.
A Home Office spokesperson said: "Drug misuse has a serious impact on society and the ban on khat will help protect vulnerable members of our community.
"It will also prevent the UK from becoming a single regional hub for criminals trying to make a profit, as countries across Europe have already implemented the same ban.
"Parliament has now approved the government's decision and khat will become a class C drug on June 24, 2014."
Users of the plant claim they are being unfairly targeted.
Mahamud Ahmed Mohamad - who owns the UK's largest khat warehouse in west London - insists that chewing khat is a Somali tradition.
He told Sky News: "It's like closing a pub ... will you feel happy if you close British pubs?
"Why don't you close alcohol which is affecting a lot of people? Why is it only khat that is a major issue?"
He currently employs around 40 workers and says they will be made redundant once the ban is implemented.
Mr Mohamad is challenging the ban in the Court of Appeal.
Abukar Awale, a former user, insists the substance is addictive and psychologically damaging.
The anti-khat activist blames his former habit for a violent confrontation during which he was stabbed.
"Availability of khat and the legality of khat was attracting more young people," he said. "By banning it we are preventing young people from failing in society."