IUK was established in 2010 to support major infrastructure projects involving public sector capital, such as Crossrail.
The MPA was established in 2011 and supports the projects comprising the Government Major Projects Portfolio, some 200 projects totalling nearly £500 billion in public spending.
A statement from the government said the new organisation "will bring together government expertise in the financing, delivery and assurance of these projects, which range from large scale infrastructure projects such as Crossrail and the Thames Tideway Tunnel to major transformation programmes such as Universal Credit."
It will come into formal existence on 1 January 2016.
Chancellor George Osborne said: "By bringing together Infrastructure UK with the Major Projects Authority, and creating the new National Infrastructure Commission, we are moving to the next stage in our plan to ensure Britain’s economy gets the transformational projects it needs."
The National Infrastructure Commission (NIC), established in October, will assess the UK’s infrastructure needs every five years.
