The role of Cabinet Secretary is expected to be diluted with Downing Street assuming some of its responsibilities, in the latest shake-up of Whitehall. Simon Case, permanent secretary in Number 10, would reportedly take on some of the responsibilities of Sir Mark Sedwill, the outgoing Cabinet Secretary, under plans understood to be favoured by Dominic Cummings, the Prime Minister’s chief aide. While the Cabinet Secretary will continue to head the civil service, officials responsible for dealing with other government departments could report to Mr Case instead. Meanwhile a new “delivery unit” is expected to be set up in the Cabinet Office as part of Mr Cummings’ drive to speed up policymaking. Sir Mark stepped down as Cabinet Secretary and National Security Advisor on Sunday, amid a continued government shake-up of Whitehall. David Frost, currently the UK’s chief Brexit negotiator, will replace Sir Mark as National Security Adviser, while a new Cabinet Secretary is yet to be appointed. Mr Case, formerly Private Secretary to the Duke of Cambridge, was brought in to Number 10 as permanent secretary during the Covid crisis. Downing Street sources this week said Mr Case is expected to return to the Palace when he leaves Number 10. However a source told the Telegraph Mr Case may expand his role by taking on further control of the Cabinet Office from his current position in Number 10, under plans understood to be favoured by Mr Cummings. The source added that Michael Gove, the minister for the Cabinet Office, is also expected to play a key role in the restructuring of the department. Mr Gove sits on a new Domestic and Economic Strategy committee, alongside Rishi Sunak and the Prime Minister, who chairs it. The head of the Economic and Domestic Affairs Secretariat, who currently reports to the Cabinet Secretary, would report to Mr Case instead, according to plans reported in the Spectator magazine. The new delivery unit would also report to Mr Case under the plans. Alex Thomas, programme director at the Institute for Government think tank and former civil servant, this week told the Telegraph the Cabinet Office currently includes a “series of particular units, like on delivering projects and experts on the constitution. What doesn’t exist is a powerful delivery unit that sits in the centre and holds departments to account to get things done.” The departure of Sir Mark as the country’s most senior civil servant followed rumours he had been sidelined by Boris Johnson and Mr Cummings. In a recent briefing with government aides, Mr Cummings reportedly detailed the shortcomings of the “incoherent” Cabinet Office. However he dismissed as “media inventions” reports that he wanted to centralise power in Number 10, instead saying the intention is to create a “smaller, more focused and more elite centre”. Asked earlier this week about Number 10 taking on some Cabinet Office responsibilities for Mr Case to oversee, a Downing Street source said such suggestions were “not true”.