The Prime Minister needs to replace her former de-facto deputy, Damian Green, who she sacked nearly three weeks ago after he admitted lying about computer porn allegations.
It is also thought the PM is keen to restore some gender balance to her Cabinet and boost the number of women, as well as trying to balance the Tories' pro-Brexit and Remain factions.
:: Sky Views: Bring on the Cabinet reshuffle!
It has been suggesed that Mr Green's old job of First Secretary of State could go to the Health Secretary, Jeremy Hunt, though the current NHS winter crisis could now thwart his promotion hopes.
Another candidate who has been tipped for Mr Green's job, which involved chairing several Cabinet committees, is the Transport Secretary Chris Grayling, a long time ally of the PM.
Although he was attacked for visiting Qatar during New Year rail fare increases and by Labour peer Lord Adonis over rail franchises, Mr Grayling and Mrs May were local councillors together before becoming MPs.
Senior Whitehall sources have told Sky News they expect the four most senior members of the Cabinet - Philip Hammond, Amber Rudd, Boris Johnson and David Davis - to stay in the same job.
Mr Hammond's survival as Chancellor will dismay and infuriate Brexiteer Tories, who claim his pro-Remain sympathies are leading him to do everything he can to sabotage Brexit negotiations.
Amber Rudd is expected to stay at the Home Office, even though she is a close ally of the PM and could be a candidate to replace Mr Green, because it is claimed Mrs May sees her as protecting her own Home Office legacy.
Boris Johnson is likely to survive at the Foreign Office despite a succession of blunders. Attempts to demote him to a new Brexit role have been dropped because of fears that he would quit and become a dangerous assassin.
David Davis is said to be safe because, despite claims that he has grown weary and also fears he is being sidelined in Brexit negotiations, to move him mid way though his battles with the EU's Michel Barnier would cause massive disruption.
It is known that Sir Patrick McLoughlin, widely blamed for the Tories' poor general election performance in June, is to step down as Conservative Party chairman. The favourite to succeed him is the Home Office minister Brandon Lewis, who already attends Cabinet and is a local government protege of Tory chairman Sir Eric Pickles.
A fierce battle is raging, however, over the future of the Education Secretary Justine Greening, a comprehensive-educated opponent of the PM's flagship policy to bring back grammar schools.
Ms Greening, along with Business Secretary Greg Clark and the Leader of the Commons Andrea Leadsom, are thought to be vulnerable, but could be shuffled to new jobs rather than sacked.
The Sunday Telegraph reports that Ms Greening is fighting to hold on to her position, posting a series of tweets highlighting her achievements and twice declaring "school standards are rising".
Mrs May is known to reward ministers she believes she can trust and who are reliable, which could mean promotion for allies like the Northern Ireland Secretary James Brokenshire and the Culture Secretary Karen Bradley. Both were junior ministers under Mrs May at the Home Office.
After Cabinet changes on Monday, the PM is expected to freshen up the junior ranks of the Government on Tuesday, letting go of some older and long-serving stalwarts and bringing in some MPs elected in 2015.
The Prime Minister is also likely to promote a number of women MPs and some from ethnic minorities, in a bid – it is claimed - to give the Government a more modern image that better reflects UK society as a whole.