A child with British nationality is believed to be among those unaccounted for following the Spain terror attacks, Theresa May has said.
The Prime Minister said UK authorities were "urgently looking into reports" that the child, who has dual British nationality, is missing.
She spoke after the grandfather of Julian Alessandro Cadman made a plea for information about the boy, who became separated from his mother during the attack in Barcelona.
Tony Cadman, whose Facebook profile says he lives in Sydney and is from Gillingham, Dorset, posted a photograph of Julian, writing: "My Grandson, Julian Alessandro Cadman is missing. Please like and share."
The death toll from the atrocities in the Catalan capital and later in the town of Cambrils has risen to 14, with "a number" of Britons believed to be among the 130 people injured.
Speaking from her country residence Chequers, the Prime Minister said: "The UK stands shoulder to shoulder with Spain in confronting and dealing with the evil of terrorism, and I have offered any assistance we can provide.
"Sadly I must tell you that we do believe that a number of British nationals were caught up in the attack and we are urgently looking into reports of a child believed missing, who is a British dual national."
Mrs May spoke to reporters after authorities in Spain revealed they had made a fourth arrest in connection with the attacks, which killed or injured people from 34 different countries.
Three people are in custody in connection with Barcelona but a manhunt had also been under way for Moussa Oukabir, 18, El Pais said.
The newspaper reported that he is the teenage brother of Driss Oukabir, one of the men already detained.
Mrs May, who spoke with Spanish Prime Minister Mariano Rajoy on Friday morning, added: "We must work together if we are to confront this evil of terrorism, and also to confront and deal with the perverted extremist ideology which drives it - ridding the internet of poisonous material, and ensuring that our police and security services have the powers that they need.
"Terrorism is the great threat that we all face, but together we will defeat it."
Barcelona, a hugely popular tourist destination, came to a halt at noon on Friday (11am BST) as a minute's silence was observed in the Placa Catalunya, close to the scene of the attack.
Led by King Felipe and PM Mr Rajoy, the silence was then followed by applause for the victims.
Three days of mourning have been declared by the government of Catalonia.
The Foreign Office said a "small number" of Britons had been injured and extra staff were being sent to Barcelona to help.
After 13 people were killed in Barcelona, a woman died in hospital on Friday, hours after being injured when a car carrying five men wearing fake suicide belts ploughed into pedestrians in Cambrils.
All five terrorists in the town, 70 miles (113km) south-west of the city, were shot dead by police.
In a chilling echo of the London Bridge attack in June, Catalonia president Carles Puigdemont said the five terrorists in the vehicle were wearing fake suicide belts when they were stopped, and linked them to the earlier Barcelona attack. Both attacks involved vehicles being driven at pedestrians.
Mr Rajoy said the fight against terrorism was "one of the greatest problems that affects all of the European Union".
He added: "What we have said today is the will, that I consider fundamental at the moment, to work together to generate trust, because that is what will take us to greater efficiency which is what we are looking for. "