Jill Rutter is a Programme Director at the Institute for Government.

Top of the new Prime Minister’s in–tray will be putting in place the arrangements for organising the UK’s exit from the European Union. Disentangling 40 years of increasing engagement with Europe is one of the biggest collective tasks Whitehall has faced in peacetime.

David Cameron put in place a holding arrangement: a Brexit unit in the Cabinet Office, reporting to the whole Cabinet and overseen by Oliver Letwin. Theresa May will want to put in place more permanent arrangements. So far she has said she will appoint a leading Brexiteer as a Secretary of State for Brexit, and earlier she pitched the idea of a Ministry for Brexit.

Appointing a Secretary of State for Brexit is a good idea. Without a heavyweight lead who clearly has her confidence, too much will land straight on the PM’s desk – whether it is banging heads together in Whitehall, mollifying external players or the very active diplomacy that will be required to secure the government’s desired outcome. The PM does not want to have to expend her political capital and time too frequently on issues that can be resolved without her intervention.

That job could be combined with another department. The Foreign and Commonwealth Office and The Treasury would be the two most obvious. Liam Fox suggested a refocused foreign and trade ministry and Michael Gove appeared early on to be angling to run the negotiations as Chancellor. But each department has its own drawbacks: The FCO has little existing capacity for the coordination of domestic policy (a huge element of the Brexit challenge), and The Treasury has its own vested interests and would be viewed with suspicion by other departments. Either would need to coordinate with the Cabinet Office who already coordinate across Whitehall and support prime ministers in their role as diplomat in chief. That double coordination looks too inefficient.

That points to building up the existing Cabinet Office structure. A Brexit unit needs to be able to draw on expertise from across Whitehall and outside. It needs to have its own strong analytic capacity and ability to challenge departments to make sure ministers can take decisions on the basis of the best advice possible. And the new Prime Minister will need to make sure she has the internal processes to bind in her senior Cabinet colleagues to the decisions being made – a European version of the National Security Council.

That could form the nucleus of a future new Whitehall department – to create a power base for the new Secretary of State. But all experience of machinery of government changes says that creating a new government department is a costly distraction – senior management gets bogged down in sorting out the logistics of IT, HR and budgets and cannot focus on the job in hand. With our European counterparts rattling the cage to know what the UK option is, this is no time to divert the attention that needs onto the logistics of sorting out the email addresses for a new department. Better to leave a decision on whether to create a new department for the later phases of implementation – and only do it if there is a clear and compelling business case.

So the message to Theresa May is to build on and build up what David Cameron began to construct; make sure that the civil service can get people with the right skills when they are needed; and, crucially, to put in place the political personnel and processes to ensure that collective decisions are made rigorously but quickly and executed effectively.