Britain has issued its sharpest condemnation of the direction of Saudi Arabian human rights policy, describing its mass executions as “repulsive” and “utterly unacceptable in the modern world”.

The remarks came after further details emerged of the Saudi government’s execution on Tuesday of 37 people, including three who were minors at the time of their alleged offence.

One of those executed was then crucified, according to Saudi state media.

The Foreign Office minister Sir Alan Duncan, answering an urgent question in the Commons, spurned the usual diplomatic niceties, saying the mass executions were “a deeply backward step which we deplore”. He added it was “deplorable and totally unacceptable” that at least one of those executed had been a minor at the time of the arrest.

He highlighted reports that one of those executed was displayed on a cross, saying that anyone in the House, just two days after Easter, would find “more repulsive than anything we could picture”.

He added: “Any country needs to realise that when it uses methods like this they will eventually backfire. The practical benefit is entirely negative.”

The Treasury minister Liz Truss said there needed to be a review of UK policy towards Saudi Arabia, while Labour MPs called for the country to be stripped of the right to host the G20 summit next year.

Duncan said the Foreign Office would seek details from Riyadh of the crimes of those executed and the due process, but added the UK had been denied access to some trials in Saudi Arabia.

British diplomats have been allowed to attend the trial of those charged with killing the Washington Post columnist Jamal Khashoggi in the Saudi consulate in Istanbul, but no public report has been provided.

Thirty-three of the 37 executed in six cities on Tuesday were Shia Muslims, often suspected by Saudi authorities of being loyal to Iran.

Duncan said: “The broader picture does give growing cause for concern if you look at those executed – the number, the Shia, the minors, those whose crimes we do not know, the Khashoggi incident, we will be very robust in the representations we make in the embassy and at a minister to minister level. It is very important that the regime in Saudi Arabia appreciates that world opinion can only get louder in its condemnation.”

He extended his criticism to the Saudi-led war in Yemen, saying: “Bombings in Yemen do not really achieve any of the objectives they have set out to achieve and we need a political settlement as a matter of urgency.”

But he resisted cross-party calls from MPs, including Conservatives, for a fundamental review of Britain’s relations with Saudi Arabia. “The UK had to be aware of the entire Gulf and the dangers around it,” he said. “There is a moral dilemma here. There is deep murkiness here that we do not like.”

He also equivocated on whether Britain would continue to support Saudi Arabia sitting on the UN human rights council.

Further condemnation came from the UN human rights chief, Michelle Bachelet, who described the executions as shocking.

The EU’s diplomatic service said the killings confirmed a negative trend in the country, in contrast to the decline in death penalties worldwide.

Amnesty International said of those executed, 11 men were convicted of spying, and 14 others were convicted of violent offences, including participation in anti-government demonstrations in Saudi Arabia’s Shia-majority Eastern Province between 2011 and 2012. The 14 were subjected to prolonged pre-trial detention and told the court they were tortured or otherwise ill-treated to extract “confessions” from them.

Despite the killings, the Saudis went ahead with a major investors conference in Riyadh, setting out a plan for its chairing of the G20 and for the Saudi capital market to be among the top 10 in the world by 2030.