Ministers are unable to explain what is meant by the Prime Minister’s phrase for maintaining the UK’s international prowess outside the EU, the highly critical report said.
To remove the “confusion”, MPs called on Ms May to make a landmark speech in the House of Commons to finally “set out what the policy means”.
All documents, speeches and policy work should be gathered in a single place – in all the world’s leading languages – to give it substance, the Commons Foreign Affairs Select Committee said.
“For global Britain to be more than a worthy aspiration, the slogan must be backed by substance,” its report warned.
“If it comes to be perceived as a superficial branding exercise, it risks undermining UK interests by damaging our reputation overseas and eroding support for a global outlook here at home.”
At present, the phrase amounts to “little more than a continuation” of the Foreign Office’s current activities, the MPs said.
The Prime Minister first used the “global Britain” in her Tory party conference as long ago as October 2016, when she insisted the UK would “forge an ambitious and optimistic” role in the world after Brexit.
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Boris Johnson, the Foreign Secretary, is among senior ministers who have also used the phrase in major speeches.
But Britain’s international leverage is in question after the withdrawal of a candidate to fill a vacancy on the International Court of Justice (ICJ) last November, due to inevitable defeat in the UN General Assembly.
Before Christmas, a former UK ambassador at the UN said he expected the EU to also refuse to back Britain in any UN votes over the Falkland Islands after Brexit.
Other countries regarded the UK’s loss of influence since the Leave vote “as a shark would regard blood in the water”, Lord Hannay warned.
And Sir Simon Fraser, a former head of the diplomatic service, accused Ms May of “mushy thinking”, calling her global Britain ambition just a “slogan”.
Other countries believe Britain has “lost the plot” by pursuing Brexit because it will reduce the country’s “international influence”, Sir Simon told the committee’s inquiry.
The committee announced a further inquiry into what is “required to make global Britain a credible strategy” and how the success of it should be assessed.
It would also explore how “foreign policy such as rule of law, cultural influence and military interests should be balanced against the trade aspects of global Britain”.
And it would ask “which regions, countries and multilateral organisations the UK should prioritise” as well as whether it has the capacity to do that.