THERESA May has insisted her Brexit vision in which Britain will align with European Union rules to get "frictionless" trade but retain the freedom to diverge from them is "ambitious","credible" and "practical".

The Prime Minister also claimed voters were tired of politicians arguing over Brexit and so aimed to be "straight with people" in her high-profile speech at London's Mansion House on Friday.

The details of her Brexit strategy appeared to unite warring Remain and Leave factions in the Conservative Party.

READ MORE: Point of principle at stake over EU withdrawal bill, says Nicola Sturgeon

But Downing Street and Boris Johnson have denied reports that Mrs May's chief of staff Gavin Barwell was behind a leak of the Foreign Secretary's private thoughts on the contentious issue of the Irish border, which is holding up Brexit negotiations.

On Friday, Mrs May rejected "unacceptable" EU proposals to retain customs union arrangements in Northern Ireland but acknowledged the UK's "responsibility" to help maintain a soft border with the Republic, spelling out in detail how she believed this could be achieved by technological means or through a broader trade agreement.

But she had been under pressure to back an EU-UK customs union amid a now-stalled rebellion from Tory Brexiteers and a shift in Labour's position announced earlier in the week by Jeremy Corbyn.

In the lead-up to the speech a memo from Mr Johnson to the PM was leaked, showing the Foreign Secretary believed the UK Government should focus on stopping the Irish border becoming "significantly" harder, reigniting a row over the issue.

Mr Johnson said "even if a hard border is reintroduced" on the island of Ireland, the vast majority of goods would not be checked.

READ MORE: Point of principle at stake over EU withdrawal bill, says Nicola Sturgeon

Sunday newspaper reports that Mr Barwell leaked the memo to stop any rebellion over Mrs May's Brexit plan from the Cabinet's leading Brexiteer were strongly denied.

A spokesman for Mr Johnson said: "This is a yawnerama, a nonsense claim from an anonymous quote about a half-baked and misleading leak that has been totally superseded by the Prime Minister's excellent speech on Friday.

"We all now have a song to sing on Brexit and we are going to be in unison.

"We are coming out of the customs union and single market and as Theresa May has spelt out we can stay economically and politically close to our friends and partners in Europe while forging an exciting new future for Britain, controlling our own laws and doing our own free trade deals.

"And the PM was absolutely right to be optimistic about the solutions to the issues raised by borders in Ireland and elsewhere. It's time to be positive folks," he declared.

A Number 10 source added: "As Boris himself has made clear, these claims are ridiculous and totally untrue."

Meanwhile, Mrs May admitted unravelling Britain's relationship with the EU was "complex" but insisted she had set out a "very practical" way to move ahead in negotiations.

She told BBC One's Andrew Marr programme that her keynote speech showed she was "being straight with people" and noted: "Most people, most of the time...feel the time for arguing, either side of the referendum, is gone."

READ MORE: Point of principle at stake over EU withdrawal bill, says Nicola Sturgeon

Last week, Sir John Major, the former PM, warned Mrs May any hardening of the Irish border could lead to the "age-old" violence of the Troubles flaring up and said the issue "cannot be taken for granted".

He accused her of "unconvincing" and "easy soundbites" about finding a way to avoid a hard Irish border through technology, without offering any practical solutions.

Writing in the Mail on Sunday, he said: "What happens if local incidents cause the border to be attacked - as there is ample past reason to believe it might be? Is security brought in? Does that reactivate old disputes that begin a downward spiral? As the age-old conflicts of Ireland tell us, this is a clear and present fear. The removal of hope is corrosive."

He went on: "I know that, after the many years of peace Ireland has enjoyed since the Good Friday Agreement, it is easy to believe there is no further risk.

"And I wish that were so. But that peace, which many people strove for years to achieve, is still fragile, and even the smallest risk must not be taken," he added.