Brennan will replace Sir Bill Jeffrey, who is to retire at the end of October. She will take up her new post on 1 November.

Before joining the MOD in 2008, Brennan was director general for corporate perfomance in the Ministry of Justice. She joined the civil service in 1975 and spent most of her career in what is now the Department for Work and Pensions.

In a message to staff, Brennan said: "It is a privilege to be taking over from Bill Jeffrey at this critical time for the MOD. As 2nd PUS I have seen the passion and commitment which defence inspires in our staff.

"We'll need all of that to deliver the outcome of the Strategic Defence and Security Review and I look forward to working with you on the challenges ahead."

Jeffrey said Brennan is well-known and highly respected in the MOD and he's “confident that the department will be in capable hands as we move to implement the Strategic Defence and Security Review over the next few years.”

Cabinet secretary Sir Gus O'Donnell echoed Jeffrey’s confidence that Brennan has the “skills, knowledge and experience to lead the department to rise to the challenges it faces”. He thanked Jeffrey for "the outstanding role he has played in leading the Ministry of Defence over the past five years, and for his lengthy and accomplished record of public service."

Written by CSW