The UK must “rediscover the strategic importance of Northern Ireland to its national security” following the restoration of Stormont, a major report has warned.

Think tank Policy Exchange said there is a threat of Russia, Iran and China using the Republic of Ireland as a “backdoor” to the UK. Its report said the risks were increased due to the “Republic’s reluctance to invest sufficiently in its military and security apparatus”.

Researchers warned an expansion of Russian, Chinese and Iranian presences in the Republic of Ireland “signals the intent to infiltrate, and interfere in, the transatlantic community”.

They call on the UK Government to restore its active naval and air presence on the western side of the Irish Sea to fend off external threats.

In a foreword to the report, Defence Secretaries Sir Michael Fallon and Lord Robertson of Port Ellen welcomed the research, which they said “powerfully reasserts the strategic importance of Ireland, and especially Northern Ireland, to the UK’s national security”.

“Russian intelligence ships and warships have been identified off the Irish coast and close to key transatlantic cables,” they continued.

“The growing Russian, Iranian and Chinese presence in the Republic poses a backdoor threat to the United Kingdom itself.”

Policy Exchange author Marcus Solarz Hendriks said: “With Russia increasingly probing the vulnerability of transatlantic maritime infrastructure, the UK must take action to police its northwestern waters.

“The UK cannot do the job of the Irish state for it but – by fundamentally changing the nature of Northern Irish security arrangements – it will send a strong signal to the Republic that our patience for its evasive commitment to collective security has worn thin.”

In an historic moment on Saturday, the Vice President of Sinn Fein, Michelle O'Neill, became the first republican First Minister after a two-year political paralysis at Stormont came to an end. In her first UK interview since being sworn in, she said a referendum on uniting Ireland will be held within the next 10 years.

Asked whether she anticipated a referendum on Irish unity, Ms O’Neill said: “Yes. I believe we're in a decade of opportunity. And there are so many things that are changing all the old norms, the nature of the state, the fact that a nationalist republican was never supposed to be First Minister. This all speaks to that change.”

Asked whether she will be more conciliatory now that she is First Minister, Ms O’Neill told Sky’s Sunday Morning with Trevor Phillips she will "consider every invitation that comes my way". "I think that's important. And again, that comes back to the demonstrating, in terms of your words and deeds, that you're going to fulfil the commitment or the promise that I've made in terms of for all," she said.