Exclusive: Meetings of Theresa May's National Security Council were repeatedly put on hold because the government became "consumed" by Brexit, the prime minister's former national security adviser tells Business Insider.

Sir Mark Lyall Grant tells BI that May's attempts to forge a new "global Britain" are stalling because of the all-consuming nature of Britain's exit from the EU.

"Meetings were cancelled at the last minute because there had to be another meeting on Brexit," he said.

LONDON — Theresa May's government has been forced to cancel crucial meetings of its National Security Council because it has become "consumed" by Brexit, her former national security adviser has told Business Insider.

Sir Mark Lyall Grant, who was the national security advisor to the prime minister until April last year, told Business Insider that meetings of the National Security Council were repeatedly put on hold due to Britain's exit from the EU.

"Theresa May was very clear that she wanted to continue meetings of the National Security Council on a regular basis [after the EU referendum] but there began to be more disruption," he said.

"Meetings were cancelled at the last minute because there had to be another meeting on Brexit," he said.

The National Security Council, which is chaired by the prime minister, meets in order to discuss how to deliver the government's national security objectives, including its response to terrorist attacks and other external threats.

Lyall Grant said that May's ambition for Britain to be an outward-looking country outside the EU was also being hampered by the fact that the government is now "consumed" by Brexit.

"The government is so consumed by Brexit, and there is a risk this will continue well beyond March next year," he told Business Insider.

"That means you could lose your opportunities to develop your "Global Britain" agenda, and that in turn could have an impact on your national security, writ large," he said.

He said that meetings with other world leaders had been held back because of the all consuming nature of Brexit.

"The ability and time available to the PM to meet foreign leaders and to travel has also been more curtailed in recent years than it has been in the past," he said.

He pointed to the UK's absence from a summit meeting on Syria this week — attended by the leaders of France, Germany, Turkey, and Russia — as an example of the UK abstaining from its traditional global leadership role because the government is so consumed with Brexit policy.

"The government has set out its ambitions that we will not be insular, and that is good and right and proper," he said.

"Nonetheless, you can get insularisation by default if fewer of your ministers are travelling overseas, and you're not able to be involved with the big summit meetings."

However, he said that Brexit was unlikely to have a significant direct impact on the UK's national security or its international standing, because neither depended on Britain's membership of the EU. Much more important, he said, was Britain's participation within non-EU agencies such as NATO, the UN Security Council, and the Five Eyes intelligence community.

Nonetheless, he said that the lack of "political bandwidth" as officials focus on Brexit at the expense of other policy areas could hamper the government's stated goal to maintain its international standing.

The government's opponents accused May of taking her eye off the ball on national security.

"These words from someone so senior and who worked so closely with the Prime Minister should be a wake up call for Downing Street," Liberal Democrat MP and Best for Britain campaigner Layla Moran said.

"It seems it's not just schools, the NHS and children's services that are falling off the agenda but also the vital work of national security...

"The government have been consumed with trying to turn this pigs ear Brexit into anything that looks borderline acceptable to the public, even though it just won't work. The civil service is being stretched towards breaking point by this ongoing car crash Brexit."

'We are on the positive side of the ledger'

Since leaving government, Lyall Grant has taken up a position as a senior advisor at CTD Advisors, a firm which provides strategic advice to firms expanding into emerging markets, and he said that Brexit provided an opportunity for firms to build trade links outside Europe at a time when firms are placing "an increasing focus on the rest of the world."

He also remains confident that Britain will strike a deal with Brussels, because Britain is a "net contributor" to European security and because he said it was in both parties' interests that the UK continued to participate within information-sharing agencies such as Europol, the Schengen Information System, and the European Arrest Warrant.

"We are on the positive side of the ledger," he said.

"European leaders understand and recognise that.

"By the time negotiations come to and end, there will undoubtedly be a deal that will allow cooperation to continue because it is in the Europeans' interests and it is in our interests."

Downing Street were contacted for comment but did not respond by the time of publication.