A Saudi court has sentenced 15 people to death for spying for Iran, Saudi-owned media has reported, in a ruling that could further stoke tension between the two rival powers.

The specialised criminal court in Riyadh sentenced 15 other suspects to prison terms ranging from six months to 25 years, and acquitted two, the Arabic-language al-Riyadh newspaper reported.

The suspects – 30 Saudi Shia Muslims, one Iranian and an Afghan – were detained in 2013 on charges of spying for Iran and went on trial in February. The rulings are subject to appeal, and death sentences must go to the king for ratification.

The trial is the first in recent memory in which Saudi citizens have been accused of spying and comes at a time of high tension between Saudi Arabia, the regional Sunni powerhouse, and Iran, a non-Arab Shia theocracy, over influence in the Middle East.

In January, Saudi Arabia executed prominent Shia cleric Nimr Baqir al-Nimr, who was convicted of involvement in the killing of policemen, prompting protesters to storm the Saudi embassy in Tehran. Riyadh then broke off diplomatic ties.

Many of the suspects are former employees of the Saudi defence and interior ministries, Saudi media said. They were accused of setting up a spy ring and passing sensitive military and security information to Iran, seeking to sabotage Saudi economic interests, undermining community cohesion and inciting sectarian strife.

The charges also included supporting protests in the Shia-majority region of Qatif in Eastern province, recruiting others for espionage, sending encrypted reports to Iranian intelligence via email and committing high treason against the king.

Among those arrested in 2013 were an elderly university professor, a paediatrician, a banker and two clerics. Most were from al-Ahsa, a mixed Shia and Sunni region that is home to about half the members of the kingdom’s minority Shia community.

The timing of the Saudi court announcement is deeply embarrassing for the UK prime minister, Theresa May, who is currently on a two-day visit to Bahrain where as guest of honour she will meet the six members of the Gulf Co-operation Council and hold bilateral talks with King Salman of Saudi Arabia. The British foreign secretary, Boris Johnson, is also due to visit the region soon.

May has already faced criticism from human rights groups for her willingness to set aside human rights concerns in the Gulf to pursue defence contracts and seal free trade deals following the Brexit vote. The Foreign Office campaigns against the death penalty as one of its human rights objectives.

There will also be astonishment in diplomatic circles that the Saudis did not realise the timing of the announcement would deepen the controversy over her visit and the warmth of her contacts with the Saudis.

The Saudi foreign minister, Adel al-Jubeir, has twice in the past two months come to parliament to speak to MPs to persuade them that the country is reforming and to defend the conduct of the Saudi air campaign in the civil war in Yemen. May’s government has rejected calls to end arms sales to Saudi Arabia, saying it has not been proven that the arms are being used in breach of international humanitarian law.

The human rights organisation Reprieve had written to May before the visit asking her to raise concerns about the sentencing to death of three children arrested for street protests against the king.

Reprieve claims “Saudi Arabia has sentenced to death Abdullah al Zaher, Dawoud al Marhoon and Ali al Nimr for alleged involvement in protests in the kingdom, despite their being 15, 17 and 17 respectively at the time of their arrest”.

Reprieve adds “All three were tortured into ‘confessions’ and convicted in secretive trials. They remain imprisoned under sentence of death and could be executed at any time, without even their families being informed beforehand. On 2 January, several juveniles were among 47 people executed en masse in the kingdom”.

Saudi Arabia has blamed sporadic unrest among Shias in Qatif on Iran, but has never publicly presented evidence of a direct link between Tehran and those who took part in protests between 2011 and 2013. Iran denies any involvement.

Shias in Eastern province say they face persistent discrimination affecting their ability to work, study and worship freely, though Riyadh denies this.

Relations between Saudi Arabia and Iran soured after the latter’s 1979 revolution, which brought Shia clerics to power. Saudi Arabia follows the rigid Wahhabi school of Sunni Islam in which Shias are seen as heretics.