https://www.llal.org.uk/press-release.html
. Under the London Luton Airport (LTN) Vision for Sustainable Growth 2020-2050, passenger capacity would go from 18 million to 36-38 mppa, and the airport would accommodate 240,000 annual air traffic movements.
Just to make it clear, as stated above, the true figure is 36-38 million. The only reason it was dropped was that the council found out that by going for the upper figure they would have to pay for major M1 improvements. By going for 32 million they avoid the costs but they can then give themselves planning permission to add another 4 million later.