Fears are growing that Saudi Arabia is about to behead a man for crimes committed when he was a juvenile – in violation of both international and Saudi law – triggering calls for the foreign secretary, Boris Johnson, to publicly reject Saudi “propaganda” claims that the accused should have been considered an adult at the time of the crime.

Human rights groups are concerned that a Twitter account with close links to the Saudi government is now raising the profile of Ali al-Nimr, who was sentenced to death on charges relating to his role in anti-government protests in 2012 when he was 17. In the past, similar activity on the Twitter account has been a signal that an individual is about to be executed. Two other men, Dawood al-Marhoon and Abdullah al-Zaher, convicted of similar offences when juveniles, also face beheading.

Under Saudi law juveniles cannot be executed. But, in its latest Human Rights Priority Country assessment for Saudi Arabia, the Foreign Office states that “all three were convicted of crimes committed when they were juveniles, although under Saudi law they are considered to have been adult at the time”.

The case of al-Nimr was highlighted by Labour leader Jeremy Corbyn last year and became a cause célèbre. Following Corbyn’s appeal, the Foreign Office, which raised the matter with the Saudis, said it did not expect the three to be executed. But the Foreign Office’s decision to repeat the Saudi claim has alarmed human rights groups, who are concerned about the signals it sends out to the kingdom.

“The British government should not be accepting the Saudis’ excuses for their appalling plans to behead people sentenced to death as children,” said Maya Foa, director of the death penalty team at international human rights organisation Reprieve.

“The government must stop hiding behind the fiction that Abdullah, Ali and Dawood are considered adults under Saudi law. The reality is that the Saudis are breaking both their own laws and international law in their plans to execute these three – all of whom were arrested when they were under 18 and accused of involvement in protests calling for reform.”

Saudi law stipulates that juveniles can be classed as adults if they have hit puberty and are close to adulthood. But this has to be made clear at their trial. Reprieve insists this was not the case. It points out that all three were held in juvenile detention on arrest, and has written to the Foreign Office urging it not to accept the Saudi line. It has also raised the matter in a letter to Johnson.

“The attention the Foreign Office has paid to these cases so far is welcome,” Foa said. “However, they must ensure they are not giving support to Saudi government propaganda, as they have done all too often before. Boris Johnson needs to set the record straight and call on the Saudi authorities to immediately commute the death sentences handed down to these three juveniles.”

The issue represents an urgent challenge to Johnson’s authority. As foreign secretary he must recognise the UK’s commitment to human rights – but will also understand the need to placate the Saudis, who have bought billions of pounds’ worth of UK jets, weapons and military hardware.

The three young men claim that they were tortured into confessions. Along with serious crimes such as making and throwing Molotov cocktails at police, they were convicted of an array of offences including “observing the movements of vehicles belonging to the security forces”, “buying for and distributing water to protesters” and “explaining how to give first aid to protesters”.

All three deny involvement in violent activity. There are widespread concerns that their trials did not follow due process, as lawyers for the three were denied access to the evidence against them.

There have also been claims that the case against al-Nimr is politically motivated because he is the nephew of Sheikh Nimr al-Nimr, a reformist cleric who called for an end to corruption and discrimination against minorities. Nimr was executed in a mass execution of 47 prisoners last January, including Ali Saeed al-Rebh, 18, who was arrested after attending protests when he was 17.

A Foreign Office spokeswoman said: “The government’s position is clear and understood by Saudi Arabia: we oppose the death penalty in all circumstances and in all countries. We regularly raise the case of Ali al-Nimr, and the two others who were juveniles when they committed the crimes of which they have been convicted, including during the former foreign secretary’s most recent visit to Saudi Arabia. We expect that they will not be executed. Nevertheless, we continue to raise these cases with the Saudi authorities.”