The Home Office will not strip the British-Egyptian activist Alaa Abd el-Fattah of his citizenship because his “abhorrent” past social media posts do not meet the legal bar for such a sanction, government sources have said.

Abd el-Fattah, who landed in London from Egypt on Boxing Day, has been at the centre of a political storm over social media posts he published more than a decade ago, including tweets in which he called for Zionists to be killed.

Keir Starmer said he was “delighted” by Abd el-Fattah’s arrival on Friday after the British government helped secure the activist’s release from years in an Egyptian jail. However, the prime minister has since condemned the tweets and said he was unaware of them.

Yvette Cooper, the foreign secretary, has also launched a review looking into “serious information failures” around the case, after successive Tory and Labour governments had lobbied for Abd el-Fattah’s release as a political prisoner.

The activist, who was granted British citizenship while in prison in 2021 through his mother’s birth in the UK, has apologised “unequivocally” for his posts after opposition parties called for him to be deported and his citizenship revoked.

Human rights campaigners have reacted with anger to the suggestion, saying citizenship stripping as a punishment for social media posts would be an “extremely authoritarian step”.

On Tuesday the activist faced fresh criticism from the Conservatives after his official Facebook account appeared recently to have liked a social media post suggesting “Zionists against Alaa Abd el-Fattah” were behind a “campaign” against him.

The Home Office has not officially commented but government sources said the legal bar for revoking citizenship had not been met and the evidential case in relation to Abd el-Fattah had not changed for 12 years, during which he was granted citizenship.

Shabana Mahmood, the home secretary, is unlikely to remove someone’s British citizenship unless they obtained it by fraud or are considered a terrorist, extremist or involved with serious organised crime. The decision comes with a right of appeal, as in the case of Shamima Begum, who unsuccessfully appealed against the decision by the former home secretary Sajid Javid’s decision to revoke her citizenship in 2019.

Government sources said the bar on removing citizenship was set high to provide the necessary safeguards.

Some human rights experts and campaigners have warned that allowing politicians to deprive someone of their citizenship for any reason is a slippery slope.

Chris Doyle of the Council for Arab-British Understanding, who supported Abd el-Fattah’s release, said the tweets were “appalling” but the context was that Egyptian social media was a “very angry place” at the time and it should be understood that Abd el-Fattah was not an Islamist.

Doyle added: “Politicians calling for the stripping of people’s citizenship on a bandwagon is not a path to go down. Citizenship is a right and any grounds for removing citizenship should be if there has been an improper part of the process.”

David Davis, the Conservative MP and former cabinet minister, said he was not necessarily opposed to the idea of Abd el-Fattah losing citizenship but politicians should not have that power. “We do give out citizenship too easily … and on the other hand you have spectacularly bad staff work. This problem starts with the inappropriate award of citizenship,” he said.

“That being said, it should not be in the gift of politicians whether in government or opposition to determine someone’s citizenship. That is an almost vertical slippery slope. At what point do you decide to withdraw citizenship from your political opponents or someone who is politically uncomfortable for you?

“I don’t think it’s necessarily the case he should not have his citizenship withdrawn but it should be a judicial or quasi-judicial procedure. These things should automatically go to court. It should not be a political decision.”

Steve Valdez-Symonds, Amnesty International UK’s migrant rights director, said: “Stripping anyone of their citizenship, even for hateful comments, is not a legitimate penalty and it is deeply disturbing that anyone – particularly those in or seeking positions of leadership – should have so little regard for our shared nationality that they so quickly call for such draconian action.

“Stripping someone of their citizenship because of what they may say or tweet would be an extremely authoritarian step.”

Downing Street has defended its campaign for the release of Abd el-Fattah and its decision to welcome him to the UK despite his “abhorrent” tweets a decade ago.

The prime minister’s spokesperson said on Monday: “We welcome the return of a British citizen unfairly detained abroad, as we would in all cases and as we have done in the past. That is central to Britain’s commitment to religious and political freedom. It doesn’t change the fact that we have condemned the nature of these historic tweets and we consider them to be abhorrent.”

In one resurfaced tweet from 2010, Abd el-Fattah said he considered “killing any colonialists and specially Zionists heroic, we need to kill more of them”. In 2012 he posted: “I am a racist, I don’t like white people.” He is also accused of saying police did not have rights and “we should kill them all”, and referring to British people as “dogs and monkeys”.

Downing Street said Starmer had not been aware of the past tweets until after Abd el-Fattah had entered the UK. The development raises questions about what vetting took place before Abd el-Fattah was granted UK citizenship in 2021, and what research the government carried out before it took up his case with the Egyptian authorities. Successive prime ministers, including Boris Johnson and Rishi Sunak, had campaigned for Abd el-Fattah’s release.

The Conservatives and Reform UK have both suggested the activist should be deported from the UK for the posts and have his British citizenship revoked, even though the law does not appear to provide grounds for either action. Nigel Farage has promoted a petition for people to sign in favour of deporting Abd el-Fattah to Egypt.