Britain's Foreign Secretary Boris Johnson has warned it would be "mad" to end up with a Brexit settlement that does not allow the UK to enjoy the "economic freedoms" of leaving the European Union.

Mr Johnson insisted that leaving the EU is a cause for hope not fear as he gave a speech in central London setting out aims for uniting the country as exit day grows closer.

He said leaving the EU would mean we "stop paying huge sums" to Brussels and would be able to use some of the money to fund the NHS.

It would also mean being able to take back control of borders and laws, he insisted.

He used his speech to insist that "Brexit can be grounds for much more hope than fear".

He said it would be a "disastrous mistake" to stop Brexit, which would "lead to permanent and ineradicable feelings of betrayal", adding: "We cannot and will not let it happen."

People had concerns about the strategic, "spiritual" and economic consequences of Brexit, but whatever the "superficial attractions" they could be "turned on their head", he said.

"Brexit need not be nationalist but can be internationalist; not an economic threat but a considerable opportunity; not un-British but a manifestation of this country's historic national genius." 

In a message aimed at Conservative party colleagues and pressure groups arguing for close alignment with Brussels, Mr Johnson said: "We would be mad to go through this process of extrication from the EU and not to take advantage of the economic freedoms it will bring."

Mr Johnson insisted that Britons would continue to be able to live, work and retire in the EU after Brexit.

"If we get the right deal on aviation and on visa-free travel - both of which are in our mutual interest - this expansion of UK tourism will continue, not just beyond the EU but within the EU itself," he said.

"There is no sensible reason why we should not be able to retire to Spain.

"For those who really want to make Britain less insular, the answer is not to submit forever to the EU legal order."

Mr Johnson also repeated his call for another crossing for the English Channel.

He channelled Abraham Lincoln in his Brexit address, claiming the vote had represented a desire for a Government "of the people, by the people, for the people".

He said: "Brexit is about re-engaging this country with its global identity and all the energy that can flow from that. I absolutely refuse to accept the suggestion that it is an un-British spasm of bad manners.

"It's not some great V-sign from the cliffs of Dover, it is the expression of legitimate and natural desire to self govern of the people, by the people, for the people.

"That is surely not some reactionary Farageist concept."

The speech is the first of six being made by British Prime Minister Theresa May and senior cabinet figures to set out the UK government's road map for Brexit.

Brexit Secretary David Davis, International Trade Secretary Liam Fox and Mrs May's deputy David Lidington are expected to speak in the coming weeks.

It follows criticism of Mrs May for failing to spell out Britain's Brexit aims.

Chancellor Philip Hammond, a prominent remainer who is not on the list of set piece speeches, is on a tour of European capitals aimed at building business and political ties.

Border checks raised in Dáil exchanges

Elsewhere, the Taoiseach has told the Dáil that if Britain leaves the customs union and single market, and does not replace it with a new arrangement that is very similar, then it is inevitable that there will be customs checks between Ireland and Britain.

However, Leo Varadkar said in that case Ireland would seek to trigger the backstop agreement negotiated in December, which would involve putting in place a unique arrangement for trade between the Republic and Northern Ireland.

Mr Varadkar also said Britain has asked for Ireland's assistance in negotiating a final status deal, but he said such assistance would only be provided in the context of the taskforce headed up by the EU's chief negotiator Michel Barnier.

Fianna Fáil leader Micheál Martin said he was concerned that Ireland was now being sucked into negotiations with Britain that would result in a final status agreement that will ultimately be damaging to Ireland and mean checks at the border.

He said anything less than staying in the single market involves border checks.