Emily's family described her as beautiful and bubbly
Five British women have been killed and another 12 Britons injured in a bus crash in Ecuador.
Gap-year students Indira Swann, 18, of Berks, Lizzie Pincock, 19, of Somerset, Rebecca Logie, 19, of Lancs, and Emily Sadler, 19, of Hertfordshire, all died.
Warwick-based tour company VentureCo's guide Sarah Howard, 26, of Cheshire, was the fifth victim. Gordon Brown said his thoughts were with their families.
The bus is thought to have collided with a truck near Jipijapa on Saturday.
The victims' next of kin have been informed.
Sarah Howard, 26: Cheshire
Rebecca Logie, 19: Lancashire
Lizzie Pincock, 19: Somerset
Indira Swann, 18: Berkshire
Emily Sadler, 19: Hertfordshire
Gordon Brown's spokesman said: "The prime minister was deeply saddened to hear of the deaths of five young British people in Ecuador."
The family of Emily Sadler, who was from Northwood in Hertfordshire and due to start at Manchester University in September, have paid tribute to their "fun-loving and popular" daughter and sister.
In a statement they said "her enthusiasm for adventure and new experiences had been the incentive to travel to Ecuador", and they had last spoken to her on Wednesday.
"She was a beautiful bubbly girl with her whole life ahead of her. Her loss is indescribable," they said.
None of injured Britons is thought to be in a critical condition. VentureCo said they had sustained whiplash, and minor facial and leg injuries.
A French national, who was also on the tour, and two Ecuadorians were also hurt.
A Foreign and Commonwealth Office (FCO) spokesman said the acting ambassador in Quito had arrived in Manta, where the injured were being treated.
He also said the FCO welcomed the news that Ecuadorian authorities had appointed a prosecutor to investigate the crash.
The accident happened in Sancan, on the road between Manta and Jipijapa, an Ecuadorian news website reported.
The bus was owned by Reina del Camino, a company well known to VentureCo, and the crash occurred at 1900 local time.
Ecuadorian police have said the driver of a lorry carrying a load of sand crashed into the bus. The truck driver then fled the scene.
The group was part of a 15-week "Inca and Amazon Venture", which set out on 27 March.
They had been heading to the second stage of their trip, which involved volunteering on a development project on the coast at Puerto Lopez. They were planning to renovate a children's creche there.
They had just completed two weeks of Spanish language classes in the capital Quito.
Mark Davison, a VentureCo director, said the four students were heading to university in September.
He added the group was only 30 minutes from its destination, and the accident had happened in the least dangerous, lowland part of the eight-hour journey.
"Any form of road travel in South America is inherently risky," he said. "We know this route well and we have groups there most months of the year.
"It starts in the high Andes, and this is the part of the journey covered in the morning, and then as you reach the afternoon you're onto the lowlands where the road is flatter and straighter.
"I think it was just an unfortunate combination of circumstances. It's a tragic accident. A parent should never have to go through this. As a parent myself, my heart goes out to them."
He added that the survivors were being looked after by their staff in Ecuador, the British Embassy and the Ecuadorian authorities. The group are due to fly to Quito later today for any further medical treatment.
A group set up on the social networking site Facebook ahead of the trip lists 18 expedition members, including one person from VentureCo.
About 22,000 Britons visit Ecuador every year, according to the Foreign Office.
The South American country - which includes the Galapagos Islands - is known for its volcanoes, tropical forests and rich wildlife.
The travel editor of the Independent newspaper, Simon Calder, said the country's infrastructure was basic.
"This is a third world country with all the problems that come with that," he told the BBC.
He said the operators "will certainly have gone to every length to make sure that the local bus company is reliable and safe".
"Unfortunately you cannot legislate against accidents."
Anyone concerned for the safety of a friend or relative in Ecuador can call the Foreign Office on 020 7008 1500.