The UK is urgently looking into reports that a child thought to have British nationality is missing after the Barcelona attack, Theresa May has said.
It is thought the boy could be Julian Alessandro Cadman, whose grandfather has appealed for information.
Tony Cadman said the seven-year-old had become separated from his mother during the van attack in Las Ramblas on Thursday, in which 13 people died.
Mr Cadman, who lives in Sydney, has posted an appeal on Facebook.
In addition to the 13 people killed in Barcelona, a woman died in a second vehicle attack in Cambrils, a popular seaside resort 110km (68 miles) south-west of Barcelona. Five suspected terrorists were also shot dead in the town.
A "small number" of Britons were hurt in the attacks, the Foreign Office has said.
Tony Cadman posted a photograph of Julian on Facebook and appealed for him to be found.
He added: "We have found Jom (my daughter-in-law) and she is [in a] serious but stable condition in hospital.
"Julian is seven years old and was out with Jom when they were separated, due to the recent terrorist activity."
According to his Facebook profile, Mr Cadman is from Gillingham in Dorset but lives in Sydney.
Mrs May has condemned the attacks and 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".
Although she did not identify the boy, she confirmed the UK was looking into reports of a missing child who was a British dual national.
In other developments
- The Spanish flag and the union jack are flying at half mast in Downing Street and at other government buildings
- The Queen has sent "sincere condolences" to Spanish monarch King Felipe VI, saying it was "deeply upsetting when innocent people are put at risk in this way when going about their daily lives"
- Figures from across the political spectrum have expressed their solidarity with the people of Spain
The Foreign Office has said it is "working to find out if any more [British nationals] need our help" and that the numbers of injured could rise. It also said it had deployed extra staff in Spain
It has also issued travel advice for those going to Spain.
"Our thoughts are with the victims of these terrible attacks and the people of Spain," it added.
Spain's civil protection agency said those killed and injured were from at least 34 different countries. There is a helpline - for people calling from Spain it is 112 and 012. From outside the country it is 0034 93 214 21 24 and 0034 900 400 012.
Stephanie Walton from Lincoln was in the area when the attack happened.
She describes a scene where people were running "for their lives" and recalls the sight of many flip-flops left discarded in the street.
"We ran into the café and they shut all the shutters. All I kept thinking about was the Paris and London attacks when they were coming into bars and hurting people.
"It was absolutely terrifying. When I turned around I saw bodies everywhere. It really is heartbreaking."
Spanish PM Mariano Rajoy has condemned what he called a "jihadist attack" and announced three days of national mourning.
A minute's silence was held at midday local time on Friday.