Any minute now the Prime Minister will shuffle the government. Some ministers will be promoted, some sacked, and others moved sideways. It will be a dramatic moment. It usually is. For those involved directly it is a hugely emotional and draining day – careers are made or broken, hopes and dreams fulfilled or shattered. While few outside of Westminster will recognise let alone remember who is in or out, up or down, a reshuffle is a key moment for a Prime Minister. Were they able to do what they wanted and did it go smoothly? Is Prime Ministerial authority enhanced by being able to boss their colleagues about or weakened because they could not?

For weeks inside Theresa May’s No 10 a huge amount of thought and time has been spent on who will move where. Like a game of three dimensional chess the Prime Minister has been mentally moving people around the boards, consulting the Chief Whip, her Chief of Staff and other trusted advisors, trying to maintain factional balance, refreshing without destabilising, rewarding and encouraging new talent, whilst trying to minimise offence and enmity with those she despatches. It is a tricky business and few Prime Ministers enjoy the task.

The departures of Priti Patel and Michael Fallon enabled her to promote Gavin Williamson and Penny Mordaunt straight into the Cabinet. There is more change on the way.

But this time the best job is up for grabs (assuming the Prime Minister is staying put.) That is the one recently vacated by Damian Green – the Minister for the Cabinet Office. Other jobs may seem more glamorous, may technically be more senior, might attract a bigger Ministerial car or country home, but in terms of actual power few jobs in Whitehall dispose of more power or influence.

The Cabinet Office, which celebrated its centenary last year, sits at the very heart of government both physically and metaphorically. The Cabinet Office building on Whitehall towers over No 10 and houses the office of the Cabinet Secretary, the core administrative staff of the State and the national security apparatus. In a crisis (and the Cabinet Office was born out of the crisis of the First World War) its staff provide key resource to the Prime Minister and the No 10 team. As well as providing the administrative infrastructure for the Cabinet on a day-to-day basis the Cabinet Office runs and oversees a vast amount of vital governmental work. It is the central power of modern government in Britain and of the State itself. To be Minister for the Cabinet Office is to be at the heart of government. Physically placed between the Prime Minister and the Secretary to the Cabinet Sir Jeremy Heywood.

The are two excellent books on the Cabinet Office, one Whitehall by Peter Hennessy and the other The Cabinet Office by Anthony Seldon. They are worth reading. You can also find on YouTube a series of excellent interviews by Hennessy and Seldon with every Cabinet Secretary from Robert Armstrong to Jeremy Heywood, where, understandably, apart from Sir Jeremy, they are remarkably frank about the Prime Ministers they have served. What is illuminated is how powerful the Cabinet Office is in modern Britain.

The person who becomes the next Minister for the Cabinet Office, if they play their hand well, cultivates the confidence of the Prime Minister and the trust of the Cabinet Secretary, has the opportunity to become, discretely, one of the most powerful people in the government. They will learn how things get done, see the inner workings of policy development and implementation across government, be the PM’s go to person when something needs sorting out and help ensure her writ runs across Whitehall.

Whether it is an established senior Cabinet figure like Jeremy Hunt, or very a experienced but more junior figure like Philip Dunne, Jesse Norman or Anne Milton time will tell. For whoever is made the next Minister for the Cabinet Office will be handed a golden opportunity to make a real difference at the heart of government.