The Defence Secretary has appeared to hold the door open for Saudi Arabia to join a defence partnership to build the next generation of fighter jets.

Grant Shapps was in Tokyo on Thursday to sign a treaty with Italy and Japan as part of the Global Combat Air Programme (GCAP).

The agreement, which Mr Shapps said is likely to cost “tens of billions” of pounds, will progress international collaborative efforts to build planes, called Tempest in the UK, that have supersonic capability and cutting-edge technology.

The aim is for them to take to the skies by 2035 and serve as a successor to the RAF Typhoon.

Reports emerged in the summer that Saudi Arabia had asked to join the partnership but there were suggestions that Japan was opposed to such a move.

Mr Shapps, in a press conference held in the Japanese capital after inking the new accord, defended the Middle East country’s human rights record and emphasised the long-held links between Britain and Riyadh.

He said other countries such as Sweden had previously shown interest in joining GCAP and that the UK held a defence relationship with Saudi that is “very deep and goes back many decades”.

The Cabinet minister said: “We’ve always said if other countries are interested then of course, this is an interesting conversation, whether that is Sweden or Saudi or anyone else.

“But initially this is about the three of us working together before then, together, looking to see whether there are other countries that we would work with.”

Asked about Riyadh’s human rights record, Mr Shapps said Saudi Arabia was “going through a huge change to their society”, citing reforms such as women being permitted to join the workforce and to drive a car, saying that, while they may sound “very basic” to Western ears, the moves were transforming the country’s society “in a very radical way”.

He told reporters there were “geopolitical discussions” to have before admitting potential new partners into GCAP, including looking at “where else would a country go” to buy a sixth-generation fighter jet and about “keeping countries working within the sphere of understanding of the world that we have”.

The UK has moved to restore relations with Riyadh since Crown Prince Mohammed bin Salman was accused of playing a role in the killing of Saudi journalist Jamal Khashoggi in 2018.

Prime Minister Rishi Sunak in August invited the leader for a visit to London, while former No 10 incumbent Boris Johnson travelled to the oil-rich Kingdom for talks with the crown prince in March 2022 during the global energy crisis.

But the Defence Secretary said it was “getting ahead of ourselves” to talk about Saudi joining the alliance with Japan and Italy, and appeared to recognise there could be resistance from within the trilateral partnership.

“From a United Kingdom point of view, it wouldn’t be particularly novel,” he said.

“It certainly would be from the perspective of other partners, but it is not something that is in play today either way.”

The treaty signed on Thursday confirms that the UK will host the joint GCAP government headquarters, with a Japanese chief executive at its helm at the outset.

Mr Shapps told PA news agency that it “met the logic test” for the HQ of the Tempest programme to be in the UK.

“If you remember the history of this programme — the Tempest was announced some time ago — there are about 3000 people already working on the sixth-generation jet in the UK,” he said.

“So there has been quite a lot of head start in terms of that work.

“And I think it met the sort of logic test to carry on with it with the HQ in the UK.

“And initially, for the first couple of terms, the CEO and the couple of top positions will actually be Japanese and Italian to sort of compensate, as it were, for that before it becomes open to the UK as well.”

He said “no formal decision” had been made on where the government HQ location and a separate industry base, which will also be based in the UK and led by an Italian, will be but said some areas would think of themselves as “obvious locations, given where the workforce already is”.

Joint development of the aircraft is due to start in 2025.

When complete, the Tempest will boast a powerful radar that can provide 10,000 times more data than current systems, the MoD said.

Pilots will be able to use virtual reality in the aircraft’s digital cockpit, with vital information displayed directly in front of them.

The on-board weapons system will deploy artificial intelligence and machine learning to “maximise the effect” its arsenal can deliver, Mr Shapps’ department said.

Some £2 billion has been committed to the project by the UK Government up to 2025, with the investment announced in 2021 before the partnership with the other two nations on GCAP was confirmed.

That cost is likely to grow as the project progresses, Mr Shapps suggested, with the bill likely to stretch into the “tens of billions”.

The MoD awarded the contract to BAE Systems in collaboration with Leonardo UK, missile maker MBDA UK and Rolls-Royce, as well as industry partners from Japan and Italy.