After Sir Ivan Rogers’s unexpected resignation as the ambassador to the EU, Theresa May and her aides will be manoeuvring to find a replacement for the highly experienced and respected diplomat in Brussels, who “knew what he was talking about”.

While it is likely the role will go to a civil servant with a deep understanding and knowledge of the Brussels machinery there have been calls from Brexiteers to award the job to someone who backed Britain leaving the EU at the referendum in 2016.

But speaking on BBC’s Radio 4 Today programme Sir Simon Fraser, the former head of the Foreign Office, said that the role of the ambassador and the civil servants “is to give clear, dispassionate, objective advice”. He added there will be many people keen on the role – and experienced people in the Foreign Office, the Treasury and in the Cabinet Office, who are qualified to do it.

“I think what we need in Brussels is somebody who has experience, who’s going to be a real professional negotiator, who will be sitting in a room with lots of other very experienced and knowledgeable negotiators, and who will be hold his or her own in that negotiation,” he added.

The Government, however, will have to appoint a new ambassador urgently, given the Prime Minister intends to serve the Article 50 notice – the formal mechanism to begin the exit negotiations – to EU leaders by the end of March. Sir Ivan’s resignation also comes after his deputy, Shan Morgan, announced late last year she would leave the post to be the Welsh government’s permanent secretary.

Here The Independent looks at some of the likely runners and riders to replace Sir Ivan as the next representative for the UK in Brussels:

Tom Scholar

Currently the permanent secretary at the Treasury – one of the most respected roles in the civil service – Mr Scholar also helped David Cameron, the former Prime Minister, during the negotiations with the EU last year as his principal adviser.

George Osborne, the former Chancellor, has previously described the former aide to Gordon Brown as “an outstanding civil servant, with experience of advising on some of the biggest challenges facing the country in recent years”

Oliver Robbins

The 41-year-old is a well-respected civil servant and known as impartial operator with significant EU experience. He is currently permanent secretary at the Department for Exiting the European Union and has a responsibility to support the department in the negotiations. Before this he worked as a senior civil servant in the Home Office while Theresa May was in charge of the department.

Caroline Wilson

Ms Wilson was recently appointed as the Europe Director at the Foreign Office, which means her responsibility is to promote Britain’s interests in Europe. She has previously worked in Hong Kong and Macao.

Peter Storr

The Downing Street EU adviser was brought from the Home Office – where he previously worked as the senior director for the international and immigration policy group. The Prime Minister and her closest aides have been known to respect those loyal to May during her time as Home Secretary – creating speculation Mr Storr could be in the running.

Sir Jon Cunliffe

Sir Jon, who currently works at the Bank of England as deputy governor, has done the job before – he is Sir Ivan’s predecessor as ambassador to the EU.

The Government could also consider another ambassador – or former ambassador – for the role.

Sir Kim Darroch

He is the current US ambassador based in Washington and his diplomatic career spans three decades. He has primarily focused on national security issues and EU policy.

More recently, Ms May was forced to express confidence in Sir Kim after calls from the President-elect Donald Trump to replace him with the former Ukip chief Nigel Farage. Moving him back to Brussels could also provide an opportunity for the Prime Minister to appoint a fresh face in Washington as she attempts to build a relationship with Mr Trump.

Alex Ellis