Independent experts have urged the Government not to criminalise the stimulant khat amid reports the Home Secretary is set to defy the advice of her own officials and ban the substance.

Former government drugs adviser Professor David Nutt has said that banning khat, which is popular among the Somali, Yemeni and Ethiopian communities, would show "contempt for reason and evidence".

His comments come as The Times newspaper reported that Theresa May was poised to criminalise the substance due to fears over links between its trade and Islamic extremism, as well as the effect it has on Somali communities.

Earlier this year, the Government's official advisers, the Advisory Council on the Misuse of Drugs (ACMD), which Professor Nutt used to lead before he resigned, concluded khat should not be banned.

Professor Nutt, now chair of the Independent Scientific Committee on Drugs, said: "Banning khat shows contempt for reason and evidence, disregard for the sincere efforts of the Advisory Council on the Misuse of Drugs and, most of all, indifference to the welfare and rights of the communities in which khat is used."

He added: "The Government cannot demonstrate any grounds to imagine that a ban will help rather than harm khat-using communities. They would be lying if they said they had run out of options to help the affected communities in ways that do not involve locking people up."

Around 2,560 tonnes of khat worth £13.8 million was imported to the UK in 2011/12, bringing in £2.8 million of tax revenues.

The intoxicating herb is sold in small shops and cafes in bundles worth £3 to £6 each, with most users chewing around one to two bundles in an average session lasting up to six hours.

The leaves of the product, which are chewed to obtain a mild stimulant effect, contain cathinone, which has a similar but less intense effect to cocaine.

The ACMD report found that khat has no link to adverse medical effects, other than a small number of reports of an association between its use and liver toxicity.

However, the organisation did find that excessive use among vulnerable individuals could lead to chronic physical and psychological harms but no firm conclusions had been drawn.

ACMD vice-chair Dr Hew Mathewson previously said the body paid "no credence" to reported links between the khat trade and the funding of al-Shabaab, the Somalia-based cell of the militant Islamist group al Qaida.

A Home Office spokesman said: "The Home Secretary is currently considering the advice of the Advisory Council on the Misuse of Drugs and will respond in due course."