Update - 5.51pm: In a statement, released by the Department of Foreign Affairs and Trade in Australia, his family said: "Julian was a much loved and adored member of our family.
"As he was enjoying the sights of Barcelona with his mother, Julian was sadly taken from us.
BREAKING: #Spain #Barcelona - Spanish missing persons bureau say seven-year-old Julian Cadman was one of those killed in Barcelona attack pic.twitter.com/jruFsHlRkR— Terror Today (@TerrorToday) August 20, 2017
"He was so energetic, funny and cheeky, always bringing a smile to our faces.
"We are so blessed to have had him in our lives and will remember his smiles and hold his memory dear to our hearts.
"We would like to thank all those who helped us in searching for Julian.
"Your kindness was incredible during a difficult time.
"We also acknowledge we are not the only family to be affected by the events, our prayers and thoughts are with all people affected."
Foreign Secretary Boris Johnson tweeted: "I send my sincerest sympathies to the family of Julian Cadman and all those who loved him. His death is a tragedy.
"The FCO, our Australian colleagues & the Spanish authorities continue to do all we can to support his family at this deeply distressing time."
A Foreign Office spokesman said: "We are assisting the family of an Australian-British child who was killed in the Barcelona terrorist attack.
"His family have our deepest sympathies at this very difficult time. Our staff are doing all they can to support them, working with our Australian colleagues and the Spanish authorities."
I send my sincerest sympathies to the family of Julian Cadman and all those who loved him. His death is a tragedy— Boris Johnson (@BorisJohnson) August 20, 2017
Earlier: It has been confirmed a seven-year-old boy with dual British-Australian citizenship, missing in the Spain terror attacks, was among those killed.
His father had flown from Australia to Spain in the search for his son.
It takes the total number of people killed in the twin attacks to 15.
Young Julian Cadman was separated from his mother, Jom, after a van ploughed into crowds on Las Ramblas.
She is in a hospital where she is in a serious but stable condition.
The seven-year-old's family have spoken out since the news.
"We are so blessed to have had him in our lives and will remember his smiles and hold his memory dear to our hearts."
Sky's home affairs correspondent Mark White is in Spain and said both parents are coming to terms with the news.
"His mother we are told is still in hospital and being treated for her injuries," he said.
"His father is in the hospital now and is comforting his wife and coming to terms with the devastating consequences of this terrorist attack."
Crowds leave flowers and pay their respects to the 13 victims of the Las Ramblas attack in #BarcelonaAugust 20, 2017
The news comes as a memorial mass has been held in Barcelona for the victims of Thursday's attacks that left 14 people dead.
The archbishop of Barcelona called for Spain to unite to work for a more peaceful world.
He told the congregation the terrorists would not win in their attempt to take away innocent life.
"Our silence, our prayers and our presence in this place is an expression of rejection of those attacks," he said.
The confirmation of Julian's death comes after Spanish police linked three rental vans to the fugitive at the centre of a manhunt following the attacks in Barcelona and Cambrils, which lies around 70 miles to the south west.
Authorities said the 12-strong terror cell responsible for the attacks has been dismantled, but the search is still on for Younes Abouyaaqoub, a 22-year-old Moroccan named in Spanish media as the suspected driver of the van used in the massacre on Las Ramblas.
A police official said all three vans were rented using the credit card of Abouyaaquoub, who is the focus of a manhunt in north-east Spain.
The official said one of the vans was used in the Barcelona attack, while another was found in Vic, 44 miles north of Barcelona on the road to Ripoll, where all the main attack suspects lived, and the third was found in Ripoll itself.
Police believe the cell wanted to load the vans with explosives for a big attack, but their plans changed after the house where their plot was being hatched blew up on Wednesday in Alcanar.
According to the news agency, a Spanish police official said those behind the attacks had stored more than 100 gas tanks and explosive ingredients at the house in Alcanar.
Josep Lluis Trapero said the cell had planned one or more attacks with explosives in Barcelona, and had rented three vans and also used a car and motorcycle.
He said the search for a missing fugitive has been complicated because authorities have not been able to positively identify the human remains at the house in Alcanar.
The investigation is also focusing on a missing imam who police believe could have died in that house explosion.
Spanish interior minister Juan Ignacio Zoido said five members of the cell were shot dead, four were in custody and as many as two were killed in the explosion.
He said no new attacks were imminent, the country's terrorist threat alert will be maintained at level four, and security at popular events and tourist sites around the country will be reinforced.
Meanwhile, mourners gathered for a mass in Barcelona where King Felipe VI and Queen Letizia joined politicians at the Sagrada Familia basilica to honour the 14 people killed and all those injured in the attacks.
Some 34 nationalities were among those wounded in Las Ramblas and in Cambrils.
Victims of the attack in Barcelona have been identified as British/Australian, Italian, Portuguese, Belgian, Spanish, Spanish/Argentine, Canadian and American.
The victim of the second assault in Cambrils - bringing the death toll to 14 - was identified as a Spanish woman.
The attacks took place around eight hours apart on Thursday afternoon and in the early hours of Friday.