The British Foreign secretary has warned that derailing Brexit would be a "disastrous mistake."

Boris Johnson delivered a Valentine's Day speech in London this afternoon designed to allay fears about Britain leaving the EU.

It's the first of a series of such speeches by senior British ministers aiming to show Government unity.

Mr Johnson insisted that Brexit is a cause for hope rather than fear and again claimed that leaving the bloc would mean the UK could "stop paying huge sums" to Brussels and would be able to use some of the money to fund the NHS.

He said it would mean the British Government could take back control over its borders and laws.

He denied the decision was “un-British” and “nationalist”:

“Brexit is about re-engaging this country with its global identity – and all the energy that can flow from that,” he said.

“I absolutely refuse to accept the suggestion that it is some un-British spasm of bad manners.

“It is not some great ‘V sign’ from the Cliffs of Dover.

“It is the expression of a legitimate and natural desire for self-Government of the people, by the people and for the people.”

Disastrous mistake

In a warning to pro-remain campaigners he insisted any move to reverse the process would be an attempt to “frustrate the will of the people.”

"I believe that would be a disastrous mistake that would lead to permanent and ineradicable feelings of betrayal,” he said. “We cannot and will not let it happen."

He said it was time for Brexiteers to reach out “those who still have anxieties” over Brexit.

Opportunity

He claimed the economic benefits of remaining within the EU customs union and single market are “nothing like as conspicuous or irrefutable as is sometimes claimed”

“I want to show you today that Brexit need not be nationalist but can be internationalist,” he said.

“It is not an economic threat but can be a considerable opportunity.

“It is not un-British but a manifestation of this country’s historic national genius.”

Hypocrisy

However, his Valentine's Day speech has already failed to woo pro-Remain British MPs, who have accused him of hypocrisy.

Labour MP Chuka Umunna MP, a leading supporter of Open Britain, said: "Boris Johnson is totally unqualified to preach about the perils of fear and betrayal when he engaged in disgraceful scaremongering with his ridiculous assertion that Turkey was on the verge of joining the EU and he has already betrayed millions of people by going back on his pledge to secure £350 million extra per week for the NHS.

"This is hypocrisy of the highest order.

"He has so far failed to explain why he is campaigning in Cabinet to take the UK out of the Customs Union when there is no other solution to the Irish border issue and it will jeopardise the Good Friday Agreement settlement.

"He has failed to explain why the Government has failed to start negotiating new trade deals when the campaign he led promised we would do so immediately after the Leave vote.

"Boris Johnson needs to come clean and concede that we cannot have our cake and eat it when leaving the EU. His speech is on Valentine's Day, not April Fool's Day."

Border checks

Meanwhile, the Taoiseach Leo Varadkar told the Dáil that if Britain leaves the customs union and single market without a new arrangement in place border checks with Ireland will be inevitable.

He insisted however that should that happen, Ireland would trigger the agreement negotiated in December and seek a unique trade arrangement between the Republic of Ireland and Northern Ireland.