The revamped bunker, at the former RAF Uxbridge site, played a vital role in defence during World War Two and was visited by both Winston Churchill and King George VI.
It will open to the public on Good Friday 30 March, with a new display on Britain’s air defence as well as a café and gift shop.
Daniel Stirland, senior curator: museums, archives and local study at Hillingdon Council, told Hillingdon is Here: “It’s the place where Britain’s air defence was planned and controlled during the Second World War. Without it and without the system which the bunker was a key part of, we probably would have lost the Second World War.
“It’s the only bunker of a similar nature that’s available for the public to visit anywhere in the UK. The history of the bunker is really of national significance and that’s why it is worth preserving.”
The new exhibition covers the history of Britain’s air defences and how it developed moving from the First World War to the Second World War.
Mr Stirland said: “The idea is that people will visit the exhibition and learn a little bit more about the history of the RAF and how it defends the UK. Then they will go downstairs into the original underground bunker and see the original operations room and they will have a better idea of how that all works.
"There a few additional things as well like a café, a gift shop and rooms to hire, but it’s primary purpose will be a museum.”
Mr Stirland said that Hillingdon Council is hoping to invest even more than the £6m already spent at the bunker in the future.
He said: “There are some rough plans for what we would like to do next. Within the bunker itself, the only room which is preserved as it was in 1940 is the operations room. But there are lots of other rooms down in the bunker that we would like to do historic reconstructions to so that it becomes more of an immersive historical experience for people.”
The exhibition and visitor centre will also contain a 95-seat auditorium and educational facilities for schools. It will be open from 10am – 4:30pm seven days a week.
Entry to both the visitors centre and bunker will cost £7 for adults and £5 for over 65s, with children under 16 being able to visit for free. Meanwhile, entry to just the visitors centre will cost £4 for adults and £3 for over 65s, with admission also being free to under 16s.
The bunker itself was built in 1939 and is situated 18 metres below ground. Visitors will only be able to visit the underground bunker on guided tours, which can be booked on arrival. Groups of 10 or more are advised to book in advance.
Councillor Ray Puddifoot, Conservative, Leader of Hillingdon Council, said the investment was about celebrating the “incredible national heritage we have on our doorstep”.
He said: “Our new museum will now tell the story of all those ordinary men and women who served here, doing extraordinary things in the line of duty.
“Following the council’s investment, this historic landmark of national significance will now be visited and enjoyed by generations to come.”
Councillor Tony Burles, Labour, Uxbridge South, said: “I feel that the Bunker visitors centre is a good idea and we should be remembering our heritage in a proper and respectful way and I feel it could be only good for the area.”