Theresa May will use her farewell visit to Scotland on Thursday to underpin her legacy as the "champion of the Union" and urge her two potential successors of the imperative need to prioritise the defence of the UK against the SNP's push for independence.

Her keynote speech in a central Scotland location will come just 24 hours before Boris Johnson and Jeremy Hunt address Tory members in a leadership hustings in Perth. On Friday, postal votes go out.

It is suggested that the Prime Minister in particular will target her words at Mr Johnson following his “do or die” sentiment on leaving the EU on October 31 with or without a deal. A recent poll suggested support for independence would rise to 52 per cent in the event of a no-deal outcome.

Mrs May will insist in her address that the 300-year-old Union is more than just a marketplace for goods but is a family of nations and a union of people.

She will use it to emphasise the need to “strengthen the Union,” and to this end is expected to announce a snap review of how the UK Government approaches the defence of the United Kingdom; part of a move to build up her legacy.

The Herald has been told that Lord Dunlop, the former Scotland Office Minister, has been approached to undertake the review, which will look at whether Government structures are configured to strengthen the working of the Union.

The review is regarded as being independent of government but, nonetheless, could be supported by a small team of civil servants. It is due to begin in August and be completed in the autumn with the report going to the new PM.

The Cabinet had been poised to discuss the Union at its regular Tuesday meeting but this was cancelled because Mrs May was waylaid in Brussels as EU leaders discussed the appointment of jobs to the new-look Commission. It is now expected the “Union Cabinet” will take place next Tuesday.

It is understood that there is a growing mood within Whitehall that, particularly with the possibility of a no-deal Brexit, that the Conservative Government has to do much more to strengthen the Union.

Already, Scottish Conservative MPs have been pushing for more direct funding to Scotland to show voters how much the UK Government does to build and improve the nation’s infrastructure. The SNP administration has denounced such a plan as a “power-grab” and a bid to undermine the devolved settlement.

Both leadership candidates are backing a proposal to have a Union Unit in No 10 to better co-ordinate the Government’s message across the devolved nations.

It is also believed there is a push to enhance the Scotland Office with the possibility of one or two more ministers, which will disappoint those who have been pushing for it to be abolished and those who want it to be subsumed into a greater Department for the Union.

Mrs May’s speech comes as Mr Johnson and Mr Hunt have been burnishing their Unionist credentials. The former London Mayor said today the “Union comes first,” and that it was “absolutely inviolable” while the Foreign Secretary has insisted he is a “Unionist to my finger-tips” and would “never ever” allow the UK to be broken up.