Britain signed a multi-billion-pound preliminary order from Saudi Arabia for 48 Eurofighter Typhoon fighter jets, military equipment maker BAE Systems said on Friday.
BAE said the signing was "a positive step towards agreeing a contract for our valued partner".
Announcement of the agreement to sell the Typhoon jets came from BAE Systems, the aerospace company set to produce the aircraft, and not from either government.
Shares in BAE Systems were up 2.7% on the news.
Saudi Arabia's mighty crown heir Prince Mohammed bin Salman has included Turkey in the "triangle of evil" along with radical Islamist groups and Iran, the electronic edition of the Greek newspaper Protothema reports.
The news comes after Qatar inked a deal late previous year to buy 24 Typhoon jets for some $8.0 billion.
The 32-year-old crown prince has shaken up his deeply conservative country, pushing through dramatic social reforms, including allowing women to drive, and waging a massive crackdown that the ruling family says went after corruption, but critics say simply targeted his political foes.
Saudi Arabia is targeting children in the brutal war in Yemen with a "growing sense of impunity", a charity chief has warned as the country's key power player completed a high-profile visit to the UK.
The UK government has faced criticism from opposition leaders and campaigners about selling military hardware to Saudi Arabia during the ongoing humanitarian crisis in Yemen.
"If May really cares for the rights of people being repressed in Saudi Arabia, or bombed in Yemen, then she must stop arming and supporting the brutal Saudi dictatorship".
"If agreed, this shameful deal will be celebrated in the palaces of Riyadh and by the arms companies who will profit from it, but it will mean even greater destruction for the people of Yemen", said Andrew Smith of the Campaign Against Arms Trade.
Prime Minister May defended the two countries' defence ties on Wednesday, saying all arms sales were strictly regulated.
The Eurofighter Typhoon is the most advanced swing-role combat aircraft available and there are nearly 500 in service across Europe.