Sir Mark Sedwill worked with Theresa May when she was the home secretary

Theresa May has cited the government’s crisis over Brexit to justify installing her longstanding lieutenant as Britain’s most senior civil servant without a formal recruitment process.

Sir Mark Sedwill, 54, was promoted to cabinet secretary after it was announced that Sir Jeremy Heywood, 56, who is being treated for cancer, would not return to work. Sir Mark had been acting in the role since June.

The prime minister is understood to have argued privately that the emergency of Brexit meant that an appointment was needed straight away.

She also said that foreign crises, including that caused by the murder of the Saudi Arabian journalist Jamal Khashoggi, meant that Sir Mark should keep his post as national security adviser. Labour said that the decision amounted