Foreign agents could be required to register with British authorities under a fresh crackdown on “hostile state” activity.

Home Secretary Sajid Javid said reviews carried out in the wake of the Salisbury nerve agent attack had revealed “real gaps” in existing laws.

He announced that the Government is preparing the way for a new espionage Bill.

In a speech at Scotland Yard, Mr Javid said: “Since the Salisbury attack, the Home Office has been reviewing the laws we have around hostile state activity.

“I believe that there are some real gaps in current legislation.

“We have to ensure that we have the necessary powers to meet the current and evolving threats to the UK, both domestically and overseas.

“Getting that right, and having those right powers and resources in place for countering hostile states, must be a post-Brexit priority.”

Mr Javid said the proposed espionage Bill will bring together “new and modernised powers”, giving security services the “legal authority they need to tackle this threat”.

Areas under consideration will include whether to follow other countries in introducing a “foreign agent registration” requirement.

The Home Secretary said: “There are other countries – the US is probably the best example in this space – that have something similar in place.

“By having laws like that, I think it would make it easier for us to act should we detect hostile state activity in the future.”

He also disclosed that he has asked officials to consider the case for updating treason laws.

Mr Javid said: “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 to considering that too.”

Any new measures would build on action already taken by ministers in the wake of the Salisbury attack in March 2018.

Legislation passed earlier this year introduced a new power for authorities to stop, question, search and detain an individual at UK ports and borders to determine whether they are, or have been, involved in hostile state activity.

Existing immigration powers have also been used in dozens of cases, Mr Javid said.

He said: “My message is clear: the UK is open to the world, but if you seek to do us harm, you are not welcome.”

In a wide-ranging speech on counter-terrorism and security, Mr Javid also:

– Noted that police and security services have foiled 19 “major terrorist attacks” in the past two years, comprising 14 Islamist plots and five motivated by extreme right-wing ideologies;

– Confirmed he has asked officials to urgently review whether new powers that make it illegal for UK nationals to enter or remain in “designated areas” abroad without a valid reason could be applied to Syria, with a particular focus on Idlib and the north-east of the country;

– Suggested there “may be a case” in future for considering “designating” parts of west Africa;

– Insisted that the Government only acts to strip individuals of their citizenship – such as in the case of Shamima Begum – following consideration of advice from security services, police and specialist officials.

The Cabinet minister discussed ongoing preparations for Britain’s departure from the EU.

He said that, while a “comprehensive” security partnership remains the Government’s preferred option, work is ongoing to prepare for a no-deal scenario.

Mr Javid also announced that Jonathan Hall QC has been appointed as the new Independent Reviewer of Terrorism Legislation.