Other countries believe Britain has “lost the plot” by pursuing Brexit because it will reduce the country’s “international influence”, says a former head of the diplomatic service.

Sir Simon Fraser also accused Theresa May of “mushy thinking” over her promise to achieve a new “Global Britain” outside the EU – branding it just a “slogan”.

Calling Brexit a “strategic error”, Sir Simon ridiculed No 10’s vow that leaving the EU would see Britain “strike out in the world”, telling a Parliamentary inquiry: “I don’t know what that means.”

And he warned: “To be frank with you, a lot of countries think, for the time being, that we have slightly lost the plot in terms of where we intend to go.”

In China last week, the Prime Minister repeatedly vowed to “build a global Britain”, arguing EU withdrawal would allow the UK to “be more outward-looking”, while retaining its strength on the world stage.

But, Sir Simon, the former Foreign Office permanent under-secretary, said: “Global Britain, at the moment, is a slogan. We need much more clarity of thought behind it.

“To be honest, at the moment, there is an awful lot of mushy thinking about this.”

Giving evidence to the Commons Foreign Affairs Committee, he warned: “Our leverage in international institutions is going to be weaker once we are outside the European Union.”

The Foreign Affairs Committee has launched an inquiry into the Government’s hopes for a “Global Britain”, following recent evidence that Britain’s international leverage is on the wane.

The biggest issues facing UK on leaving EU Show all 8 1 /8 The biggest issues facing UK on leaving EU The biggest issues facing UK on leaving EU Post-Brexit immigration workers sorting radishes on a production line at a farm in Norfolk. One possible post-Brexit immigration scheme could struggle to channel workers towards less attractive roles - while another may heighten the risk of labour exploitation, a new report warns. PA The biggest issues facing UK on leaving EU Customs union A key point in the negotiations remains Britain's access to, or withdrawal from, the EU customs union. Since the referendum there has been hot debate over the meaning of Brexit: would it entail a full withdrawal from the existing agreement, known as hard Brexit, or the soft version in which we would remain part of a common customs area for most goods, as Turkey does? No 10 has so far insisted that “Brexit means Brexit” and that Britain will be leaving the customs union, but may be inclined to change its position once the potential risks to the UK’s economic outlook become clearer. Alamy The biggest issues facing UK on leaving EU Northern Ireland-Irish border Though progress was made last year, there has still been no solid agreement on whether there should be a hard border between Northern Ireland and the Republic of Ireland. To ensure borderless travel on the island, the countries must be in regulatory alignment and therefore adhere to the same rules as the customs union. In December, the Conservative Party’s coalition partners, the DUP, refused a draft agreement that would place the UK/EU border in the Irish Sea due to its potential to undermine the union. May has promised that would not be the case and has suggested that a “specific solution” would need to be found. Getty The biggest issues facing UK on leaving EU Transition period Despite protests from a small number of Conservative MPs, the Government and the EU are largely in agreement that a transitional period is needed after Brexit. The talks, however, have reached an impasse. Though May has agreed that the UK will continue to contribute to the EU budget until 2021, the PM wants to be able to select which laws made during this time the UK will have to adhere to. Chief negotiator Michel Barnier has said the UK must adopt all of the laws passed during the transition, without any input from British ministers or MEPs. EPA The biggest issues facing UK on leaving EU Rights of EU citizens living the UK The Prime Minister has promised EU citizens already living in the UK the right to live and work here after Brexit, but the rights of those who arrive after Brexit day remains unclear. May insists that those who arrive during the transition period should not be allowed to stay, whereas the EU believe the cut-off point should be later. Getty The biggest issues facing UK on leaving EU Future trade agreement (with the EU) Despite this being a key issue in negotiations, the Government has yet to lay out exactly what it wants from a trade deal with the EU. Infighting within the Cabinet has prevented a solid position from being reached, with some MPs content that "no deal is better than a bad deal" while others rally behind single market access. The EU has already confirmed that access to the single market would be impossible without the UK remaining in the customs union. Getty The biggest issues facing UK on leaving EU Future trade agreements (internationally) The Government has already begun trying to woo foreign leaders into prospective trade agreements, with various high profile state visits to China, India and Canada for May, and the now infamous invitation to US President Donald Trump to visit London. However the UK cannot make trade agreements with another country while it is still a member of the EU, and the potential loss of trade with the world's major powers is a source of anxiety for the PM. The EU has said the UK cannot secure trade deals during the transition period. EPA The biggest issues facing UK on leaving EU Financial services Banks in the UK will be hit hard regardless of the Brexit outcome. The EU has refused to give British banks passporting rights to trade within the EU, dashing hopes of a special City deal. However according to new reports Germany has suggested allowing trade on the condition that the UK continues paying into the EU budget even after the transition period. Getty

In November, the UK was forced to withdraw its candidate to fill a vacancy on the International Court of Justice (ICJ), because of inevitable defeat in the UN General Assembly.

Last June, EU countries helped inflict a humiliating defeat on Britain at the United Nations over the legal status of the Chagos Islands, a disputed territory in the Indian Ocean.

Before Christmas, a former UK ambassador at the UN told the same committee that 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.

In his evidence, Sir Simon said the UK badly needed to give “clarity” on its foreign policy after Brexit, saying: “Other countries are looking at us in a state of some concern.”

He said it would be a “huge mistake to step away from the family of western democracies that we are so closely a part of”.

And he told the MPs: “It is hard to think of an example where the European Union has stopped us doing what we wanted to in foreign policy.

“Perhaps, in the case of Iraq, it would have been good if they had.”