Boris Johnson will announce the outcome of the Integrated Review, the Government's far-ranging review of foreign and defence policy, on 16 March.
The Prime Minister's official spokesman said it would be followed on 22 March by the publication of the Ministry of Defence's (MOD) detailed plans for the modernisation of the Armed Forces.
The spokesman said Mr Johnson had asked the Chief of the Defence Staff, General Sir Nick Carter, to continue in his role until the end of November to oversee the changes.
Gen Sir Nick, 62, who is in his third year in the post, had been expected to retire in June.
The announcement comes as Sir Stephen Lovegrove, the MOD's permanent secretary, is due to move to the Cabinet Office as the new National Security Adviser.
"General Nick has been central to setting our vision for our future armed forces," the Prime Minister's spokesman said.
"He will lead, alongside the new MOD permanent secretary, the strategy to implement the vision set out in the Integrated Review."
The Integrated Review, covering foreign, defence, security, and development policy, has been billed as the most significant overhaul of the UK's strategic posture since the Cold War.
Part of its remit has been to consider how the Armed Forces need to adapt to the changing face of warfare, with a new focus on cyber and military.
Gen Sir Nick has said it will mean giving up some "industrial age" capabilities - amid speculation that it will set out plans for a smaller Army while scrapping its fleet of Challenger 2 main battle tanks.
Cover image: PA.