Saudi Arabia has confirmed it is to buy 72 Eurofighter Typhoon aircraft, in a deal that could end up being worth more than £6bn.
The Eurofighter has taken more than 20 years to design and build
The contract, brokered between the Saudi government and the Ministry of Defence, will safeguard thousands of jobs at UK defence firm BAE Systems.
Saudi Arabia is buying the Eurofighters to replace its range of Tornado jets which were also made by BAE.
The Eurofighter was developed by BAE with European firms EADS and Alenia.
The planes will be assembled in the UK from components produced by the various partners in Eurofighter.
"The required commercial principles have now been agreed which has initiated the purchase of Typhoon aircraft and the associated commitment to the industrial plan to be launched," said the Ministry of Defence.
The ministry added that as the financial details of the deal were confidential, it could not reveal exactly how much it was worth.
Some newspaper reports have suggested it could total as much as £10bn.
The Eurofighter's UK headquarters is at Warton, Lancashire, where BAE employs 9,000 people in its aircraft division.
Confirmation of the Saudi deal, which was first proposed in December, saw BAE's shares close 2.64% higher in Friday trading in London.
Yet while the announcement is welcome news for UK business, it will inevitably attract criticism from those who say UK defence companies should not do business with Saudi Arabia because of the Saudi government's poor record on human rights and democracy.
Pressure group Campaign Against Arms Trade criticised the deal, claiming that while it had been subsidised by UK taxpayers, the Ministry of Defence and BAE had "failed to demonstrate any gains for the British public".
"The real beneficiary is the oppressive regime in Saudi Arabia, one of the world's worst abusers of human rights," it added.
The 72 planes will be assembled by BAE in the UK from parts made by all the partners in the Eurofighter project.
BAE itself makes the front and rear fuselage, while European defence consortium EADS and Italy's Alenia build the wings.
The Saudi deal is the first confirmed order for the Eurofighter to come from outside the European Union.
Inside Europe, the plane has been ordered by the UK, Germany, Spain, Italy and Austria.
The development of the Eurofighter has been hit by rows and delays, and the project going significantly over-budget.
With work first starting in the early 1980s, the maiden flight did not take place until 1994.
The plane is in competition with the French Rafale.