Theresa May has said UK authorities are “urgently” looking into reports of a missing British child.
The Prime Minister said she believed a number of British people have been caught up in the attack.
She said the missing boy had dual-nationality, but did not name him.
Earlier, appeals were launced to find a seven-year-old Australian boy who has not been seen since the attack on Las Ramblas on Thursday.
Julian Cadman had been in Barcelona with his mother, who is currently in hospital.