Proposals to renew treason laws in a bid to tackle Britons who fight on behalf of extremist groups or enemy states should be rejected, the terror legislation watchdog has said in his inaugural speech in the role.

Jonathan Hall QC, who was appointed as an independent reviewer of terrorism legislation in May, said treason laws, which date back to 1351, should be left alone.

In May, the then home secretary, Sajid Javid, said he had asked officials to consider the case for updating treason legislation. His comments came after a right-leaning thinktank, Policy Exchange, advocated renewing treason laws to prosecute Britons who betray the country through acts of terror.

However, Hall told an audience at the Royal United Services Institute (RUSI) in central London that terrorists “might welcome the risk of prosecution for treason as a badge of honour.

“Whether the mere existence of a law of treason might encourage actual terrorist attacks is debatable,” he said.

“But surely Islamic State would savour the opportunity to say that true allegiance is owed to them, not to the United Kingdom. We are in are deep waters, yet in my view, these are objections which are insufficiently answered to justify a new offence.”

Hall said a treason offence would “risk politicising and complicating criminal prosecutions with arguably limited benefits”.

The last person to be convicted under treason laws was William Joyce, more commonly known as Lord Haw-Haw, who was hanged in 1946 for assisting Nazi Germany.

The Policy Exchange report proposed using a new power for the secretary of state to “proclaim” that a state or organisation is engaged in attacking the UK in order to subject them to treason laws. Hall challenged this mechanism.

“What messages would proclaiming a group as an enemy engaged in an armed attack against the UK send?” he argued. “International humanitarian law, or the law of armed conflict, is based principally around conflict between states.

“Whilst there are some rules that apply in conflicts that are not between states, there is a natural and obvious reluctance to accord members of armed groups the same status as soldiers or combatants of a foreign state, for example, by treating them as prisoners of war rather than criminals.”

The watchdog also questioned the usefulness of the government’s heavily touted “designated area” powers, which passed into legislation earlier this year. The powers allow a home secretary to ban British citizens and residents from a specific geographic area, with a breach of a designation punishable by up to 10 years.

Hall said the powers had yet to be applied to a specific area – despite Javid announcing in May that he had asked officials to look at designating north-east Syria.

“What remains to be seen is the utility of this designation power now that the caliphate has fallen and so-called Islamic State has reverted to a more traditional clandestine terrorist group operating through networks of individuals rather than through occupation of territory,” Hall added.

The government estimates that about 900 people have travelled from the UK to engage with the conflict in Syria and Iraq, including foreign terrorist fighters who have joined Islamic State.

Speaking in May, Javid said: “I have … asked my officials to consider the case for updating treason laws. Our definition of terrorism is probably broad enough to cover those who betray our country by supporting terror abroad.

“But if updating the old offence of treason would help us to counter hostile state activity, then there is merit in considering that too.”