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Britain will lose global influence to other nations unless it can shape a new role on the world stage after the Brexit fiasco, one of the country’s most respected diplomatic figures warned today.

Sir Simon Fraser, who was top mandarin at the Foreign Office for five years, described Brexit as a bigger blow than the 1956 Suez Crisis, which became an emblem of British foreign policy humiliation.

The UK’s credibility as a country that “pragmatically conducts affairs” had been “badly damaged” by the fallout from quitting the EU, he said, adding that the country’s “pivotal” role as a link between the US and Europe had been hit.

Sir Simon said he also believes the nation’s security will be diminished.

In an interview with the Evening Standard, Sir Simon, now deputy chairman of foreign policy think tank Chatham House, emphasised that Brexit is “structurally much more important” as a setback than Anthony Eden’s disastrous attempts to regain western control of the Suez Canal because it is “reversing the national strategy” of half a century.

“Suez was just a badly handled mistake in international affairs. This is a big structural decision,” he said. While respecting voters’ reasons for voting Leave and the referendum result in 2016, he said Brexit is a “strategic error which will have costs” to the economy and Britain’s influence.

“Most countries in the world were amazed by the decision that we took on Brexit and secondly even more amazed by the way we have mishandled the delivery of that decision,” he emphasised.

“Our credibility, which used to be very high as a country which knew how to do things, and pragmatically conducts affairs, is badly damaged because of the events of the last three years.

“If we don’t succeed in clarifying Brexit, sorting the situation out, getting ourselves back on the front foot, then yes, over time our influence will be eroded and other countries will supplant that influence.”

Sir Simon led the Foreign Office from 2010-2015, led the Business Department after the 2008 financial crisis and was chief-of-staff to European Trade Commissioner Lord Mandelson.

“Essentially you exercise your leverage and influence in the world through key relationships with other countries, through your influence in international organisations — and our biggest relationships, biggest forms of leverage, have been the EU and the relationship with the US,” he explained.

To be effective internationally, countries also need “clear policies and priorities” but “Brexit has distracted us massively” from key issues, he said.

“If you look at this country, it’s difficult to identify what our foreign policy is,” Sir Simon argued. “What’s missing is clarity about what the key priority issues are,” including over China’s growing economic and political power, US foreign policy changes and migration, he added.

In trans-Atlantic talks, Britain’s voice “used to be central” as the country “interpreting” between Europe and America, continued Sir Simon, who is also managing partner of business advisory firm Flint Global.

“That is no longer the role that Britain is playing because we don’t have that pivotal sort of role,” he said.

If Brexit happens, he believes UK “leverage” with the US will decline because it will not be “representing” the EU, which he described as the “single most important multiplier of our voice internationally in recent years”.

The “instinctive” relationship with America will remain close, including on defence, intelligence and some foreign policy, Sir Simon said, but Britain is often closer to the EU on issues such as Iran and climate change.

“If you look at Brexit from the international angle... there was one strand of supporters for Brexit who were what you might call ‘little Englanders’ who wanted to sort of withdraw into a more nativist relationship with the world and another strand which was actually almost the opposite... ultra liberal free-trading so-called Global Britain,” he added.

“I don’t think we have reconciled exactly where we lie on that spectrum ... that we have thought through clearly what the image is that we want to present to the world.”

With the post-war international institutions’ authority under threat, including from a more aggressive Russia, Sir Simon said he believes it is “a very strange time” to quit the EU.

“There will be a net diminution of our security as a result of stepping outside of the EU,” he said.

While security ties will continue, including on counter-terrorism, he doubted they will have the “same quality and automaticity”.

Even if Britain decides to stay in the EU, he predicted there will be “consequences” with possible pressure on the rebate or on future opt-outs. “Leaving is difficult and staying is now difficult,” he went on to warn.

“Some of us were saying right from the beginning that this was going to take five years or a decade to sort out. We were accused by Brexiters of being remoaning mandarins... actually we were just pointing out the reality... I think we are being proven right.”