The UK has signed a £100m deal to design new fighter jets for Turkey, despite the country’s President undertaking a severe crackdown on his regime’s opponents.

Theresa May said it could open the way to billions of pounds worth of business, as she became the first foreign leader to visit Turkey since Recep Tayyip Erdogan ordered a wave of arrests and sackings in the wake of last summer’s coup.

Questioned over human rights concerns, Downing Street officials said the deal to design the TF-X jets was sealed in light of Turkey’s status as a Nato ally and claimed Ms May could approach human rights as a “separate” issue.

The PM did warn the President it was “important” for him to uphold human rights, as the stony faced Turkish leader looked on.

The UK is already mired in controversy regarding some £3bn worth of licences granted to export arms to Saudi Arabia as the Kingdom embarked on a deadly bombing campaign in Yemen.

The announcement in Ankara yesterday means BAE Systems and Turkish Aerospace Industries have signed a “heads of agreement”, establishing a partnership for the development of the Turkish Fighter Programme or TF-X.

Downing Street sources said the £100m contract has the potential to facilitate multibillion pound contracts between the UK and Turkish firms over the project’s 20-year lifetime.

Ms May added: “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.”

The failed coup last year saw Mr Erdogan enforce a state of emergency, order a wave of arrests, close numerous media outlets and remove thousands of public officials – including judges, teachers and university academics – from their jobs.

A Downing Street spokeswoman said: “Those are separate issues. Turkey is an important Nato partner and so our cooperation on both security and defence is in light of that.”

She added: “The response to the coup is that the UK has expressed strong support for Turkey’s democracy and institutions.

“But we have also been very clear that we urge Turkey's response to be proportionate and in line with international human rights obligations.”

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According to Amnesty International, the human rights situation in the country has “deteriorated markedly” since the coup, including cases of excessive use of force by police and ill treatment in detention.

The group has said Ms May's visit is a “vital opportunity” to raise concerns with the country’s increasingly powerful leader, though it was unclear ahead of the meeting if the issue would be directly raised with the President.

The UK has also signed off £3.3bn in arms exports to Saudi Arabia since the start of its offensive in Yemen, leading the Campaign Against Arms Trade to launch a High Court challenge over the legality of the sales.