Theresa May and her Turkish counterpart have announced a £100 million-plus defence collaboration deal on the continued development of fighter jets for Turkey.
The announcement comes during the Prime Minister's one-day visit to the Turkish capital Ankara, where she became the first Western leader to meet president Recep Tayyip Erdogan since last year's attempted coup.
Mrs May told the president that Britain stood with his defence of Turkey's democracy, but warned him it was "important" for him to uphold human rights.
Speaking to Mr Erdogan as they addressed the press at his presidential palace, Mrs May said: "Turkey is one of the UK's oldest friends, our relations stretch back over 400 years but there is much that we can do in the future to bulid on that relationship together.
"I'm proud the UK stood with you on July 15 last year in defence of your democracy. Now it's important that Turkey sustains that democracy by maintaining the rule of law and upholding its international human rights obligations, as the government has undertaken to do."
Mrs May had come under pressure to raise human rights issues with the Turkish president, who has imposed a state of emergency involving waves of arrests, the closure of numerous media outlets and the removal of thousands of public officials - including judges, academics and teachers - from their jobs.
Speaking before the PM's arrival in Turkey, Amnesty International's UK director Kate Allen said the visit was a "vital opportunity" for Mrs May to ask "probing questions" about allegations of excessive use of force and ill-treatment of detainees by Mr Erdogan's security forces.
The human rights situation in Turkey had "deteriorated markedly" during the state of emergency imposed after last July's botched coup, said Amnesty.
Mrs May and Mr Erdogan also discussed counter-terrorism, security, trade and migration in talks which stretched for an hour longer than scheduled.
During the visit, the UK-based BAE Systems and Turkish Aerospace Industries signed a Heads of Agreement document establishing a partnership for the continued development of the Turkish Fighter Programme (TF-X).
British officials said the contract would safeguard high-skill jobs at BAE Systems and could lead to opportunities worth billions of pounds over the lifetime of the project, with potential future contracts to provide engines, weapons, radars and sensors.
It was hoped the deal would pave the way for a deeper defence partnership, effectively making the UK Turkey's partner of choice for key aerospace technology.
Mrs May, who has made preparations for closer post-Brexit trading links her top priority for the Turkey trip, said: "This agreement underlines once again that Britain is a great, global, trading nation and that we are open for business.
"It marks the start of a new and deeper trading relationship with Turkey and will potentially secure British and Turkish jobs and prosperity for decades to come."
BAE Systems chief executive Ian King added: "The announcement signals an exciting next step in relations between both Turkey and the UK with the co-operation between BAE Systems and TAI paving the way for a deeper defence partnership.
"The agreement confirms ongoing collaborative work on the design and development of the aircraft."
The contract was announced by Mrs May with Turkish PM Binali Yildirim, following talks at his ministry building. The pair also signed an agreement on aviation security.
Mrs May was greeted at Mr Yildirim's ministry by a guard of honour and a military band playing the national anthems of both countries.
As part of the welcome ceremony, Mrs May addressed the troops in Turkish, saying "Merhaba Asker" - "Hello soldiers" - to which they bellowed the reply "Sagol", or "thank you".
Mrs May shook hands with Mr Yildirim before they entered the ministry building together.
Mrs May confirmed that Britain and Turkey are to set up a joint working group to prepare the ground for their post-Brexit trading relationship.
Mr Erdogan said he hoped to increase trade between the two countries from 15.6 billion US dollars to 20 billion (£12.4 billion to £15.9 billion).
He said he hoped to see "further co-operation", particularly in energy and defence, including predator drones, which he had discussed with Mrs May.
The pair also discussed efforts to reunite Cyprus and to tackle the threat of the Islamic State terror group in Syria.
Theresa May was determined to ensure her visit to the White House went smoothly. She had to get off on the right foot with Donald Trump, show him how keen Britain was to work with him and demonstrate that the UK is on his side. But first: the small matter of a joint news conference.