BORIS Johnson is to place defending and promoting the Union “front and centre” of his expected Cabinet reshuffle and Whitehall shake-up.

The Prime Minister is spending this weekend at his country retreat of Chequers putting the finishing touches to what will be a resetting of his One Nation Government for the post-Brexit era.

The Herald has been told that much work has been going on in determining the important relationship between No 10, the Cabinet Office and the Scotland Office and the need to instil a "Union mindset" across the whole of Government.

No 10’s Union Unit has done little or nothing to date to promote the Union. While the Dunlop Review on “strengthening the Union”, due to be published alongside the reshuffle, had originally floated the idea of a Department for the Constitution, ministers have made clear they are opposed to it.

One said: “We don’t need a Secretary of State for the Union, we already have one; he’s called the Prime Minister.”

Another insider made clear that promoting and defending the Union would be put “front and centre” in the PM’s plans. “Now that Brexit has been done, protecting the Union is the number one issue,” declared one Cabinet minister.

Given the embarrassing row this week over the crucially important COP26 climate summit in Glasgow this November, Mr Johnson is keen to steady the ship.

While he is said to be willing to forge a better relationship with Nicola Sturgeon and her SNP Government, the political reality may make this very difficult given the deep policy differences. Talk of the PM branding the First Minister “that bloody wee Jimmy Krankie woman” – strongly denied by Downing St – has not helped intergovernmental relations.

No 10’s emphasis in appointing a “big hitter” as the new COP26 President – following the rejection of the prestigious role by former Tory leaders David Cameron and William Hague - has Westminster fingers pointing in one direction: Michael Gove.

The Chancellor of the Duchy of Lancaster, second only to the PM in the Cabinet Office, is said by colleagues to want the trade portfolio so that he can oversee the crucial post-Brexit negotiations with the EU.

But the Scot, who is often in front of the TV cameras defending the Government against SNP attacks, is due next week to speak at a major green conference, raising expectations that the PM has lined him up to succeed Claire Perry O’Neill as the next COP26 President.

While there had been talk of a “big bang” shake-up of Whitehall, sources have suggested this is not likely to happen any time soon as Mr Johnson seeks to recalibrate his frontline team, bringing in more youth, female members and even Remainers to create a fresh look and balance to show that he is attempting to bring Britain back together following the years of division during the Brexit process.

Sources have suggested that the planned merger of the Foreign Office and the International Development Department is unlikely to happen in the short term and that the creation of a “Ministry for Delivery” combining trade and business will be put back for consideration later in the year.

Speculation has mounted that Rishi Sunak, the Chancellor’s deputy, had been earmarked to run the new department but will now concentrate on overseeing the autumn spending review.

Robert Jenrick, the Housing Secretary, and Oliver Dowden, the Cabinet Office Minister, have also been tipped for possible promotion.

Theresa Villiers, the Environment Secretary, and Therese Coffey, the Work and Pensions Secretary, are believed to be on the way out while Penny Mordaunt, the former Defence Secretary, who backed Mr Johnson’s rival Jeremy Hunt for the leadership, is forecast by some insiders for a return to the frontline.

Expectations are also high that Andrea Leadsom, the Business Secretary, could be another departure while Liz Truss, the Trade Secretary, thought to have been under serious threat, was name-checked several times by Mr Johnson in a speech about Brexit on Monday in London, suggesting perhaps she is safe in Cabinet; if not necessarily in her current role. Other junior ministers who might be promoted include Helen Whately, the Culture Minister, and Victoria Atkins, the Home Office Minister.

The junior ministerial shake-up that traditionally comes a day after a Cabinet reshuffle is likely to see more churn with more women and members of the new intake entering Government.

The new-look Cabinet is expected to meet for the first time on Thursday.