The UK’s most senior counter-terrorism officer has said that Shamima Begum, who left the UK to join Islamic State as a 15-year-old, had “potentially criminalised” herself as her family considers court action to stop the government blocking her return to Britain.

Government and counter-terrorism officials are still considering what to do after Begum, now 19, was discovered in a Syrian refugee camp after fleeing Isis’s last stronghold and said she wanted to return to Britain.

Neil Basu, assistant commissioner for counter-terrorism at Scotland Yard, told the Guardian the case should drive home the need to report those youngsters being radicalised, rather than let them ruin their lives after falling for terrorist propaganda.

In 2015, Begum left with two school friends from their home in Bethnal Green to join Isis in Syria. She said this week that she did not regret her decision to go to Syria, but that she was nine months pregnant and wanted to come home to “live quietly with [her] child”.

She told the Times that she expected to be charged with terrorism offences and to be the subject of intense media attention, but was desperate not to be separated from her baby.

Begum also spoke of wanting to see her family and of her fears that she might never again see her husband, the Dutch jihadist Yago Riedijk, whom she said she still loved “very much”. She said he had surrendered to fighters allied to the Syrian Democratic Forces (SDF).

Scotland Yard’s counter-terrorism command has a live investigation into Begum, which intensified after she declared her wish to return. Officials are confident that they would be forewarned of any attempt by her to fly back, through measures such as pre-travel screening, and could arrest her on her arrival.

Basu said: “We as a society must learn from this case and report signs of radicalisation early so we can stop people being radicalised, investigated by counter-terrorism policing and the security services and potentially criminalised as Begum has now invited upon herself.”

In a statement given to ITV News, the family said they were shocked by what Begum told the Times about her lack of remorse, but cautioned: “To us, those are the words of a girl who was groomed at the age of 15; we are also mindful that Shamima is currently in a camp surrounded by Isis sympathisers and any comments by her could lead directly to danger to her and her child. Given Shamima’s four-year ordeal, we are concerned that her mental health has been affected by everything that she has seen and endured.

“The welfare of Shamima’s unborn baby is of paramount concern … As a British citizen, Shamima has every expectation to be returned to the UK and be dealt with under the British justice system. Shamima’s child, who will also be British, has every right as a total innocent to have the chance to grow up in the peace and security of this home.

“We welcome an investigation in what she did while she was there [in Syria] under the principles of British justice and would request the British government assist us in returning Shamima and her child to the UK as a matter of urgency.”

The family were dismayed by the home secretary Sajid Javid’s seemingly tough talk in a newspaper interview where he said he may use powers to revoke her British passport or temporarily exclude her from entering the country – assuming she could find a way to leave the camp and get to an airport.

Their solicitor, Tasnime Akunjee, said the battle to bring Begum home could see her family take the home secretary to court: “Our view is that this would be illegal because they would make her a stateless person, in breach of international law. We are surprised the home secretary does not understand international law, or care about international law.

“If he were to attempt to put such orders on, we would explore all legal options to block his unlawful actions or appeal. If we can get an injunction, we will.”

Whether the home secretary can exclude Begum, a British citizen, from returning appears in doubt. On Friday Alex Younger, the head of MI6, said that Britons who went to join Isis were “potentially very dangerous” but cannot be stopped if they want to return to the UK.

Younger said the issue of returnees was an “extremely complex and dangerous problem”. He told journalists at the Munich security conference on Friday: “Anyone who has put themselves in this situation can expect to be questioned and investigated and potentially prosecuted, if they return to our jurisdiction.

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“We are very concerned about this because all experience tells us that once someone has been put in that sort of position, or put themselves in that sort of position, they are likely to have acquired the skills or connections that make them potentially very dangerous.”

Since 2015, the government has had the power to bar the return of jihadists under a temporary exclusion order, which can last up to two years. But legal experts have said the government would not be able to remove Begum’s British citizenship if she is not a dual national, as that would leave her stateless, which is contrary to international law. However, the home secretary has been advised that because Begum’s mother holds a Bangladeshi passport, he may be able to.

The shadow home secretary, Diane Abbott, said Labour was “not in favour of making people stateless”, but added: “If there are reasonable grounds to suspect that anyone who is entitled to return to this country either committed or facilitated acts of terrorism, they should be fully investigated and where appropriate, prosecuted. Our priority must always be public safety.”

“She was so young – I don’t think she had the life experience to make those decisions,” Mohammed Rahman, a family member, told the Times. “I think the hope would be that she would be allowed to return home, as long as the government is satisfied she has turned her back on their ideology.”