A British woman was being questioned today after her two children were found dead in a Spanish hotel room, police said.
The two toddlers were found in a room in the resort of Lloret de Mar on the Costa Brava this afternoon.
Newspaper El Mundo said on its website that the children's mother had phoned police but the children could not be saved.
The British Embassy in Madrid said an investigation had been launched into the deaths.
A spokesman said: "We are in contact with the Spanish police but because the children are minors we cannot say much.
"We believe the mother has been arrested. There is an ongoing investigation."
Embassy staff said they were trying to establish whether the family was on holiday in Spain or living in the country.
Police were called to the hotel, 47 miles north of Barcelona, at 2pm.
A woman who answered the phone at the Hotel Miramar in Lloret de Mar said she could not comment.
El Mundo reported the local police arrested the woman after the children, aged between one and five, were discovered in the hotel room where the family was on holiday.
Police arrested the woman at the four star hotel.
The mother had spoken to reception staff who called an ambulance.
Emergency services were unable to save the children, understood to be a boy and a girl.
The four-star Hotel Miramar is located on the sea front in the former fishing village, near a large number of bars and restaurants.
The family-friendly beach hotel, which has 54 rooms, is around 75km (46.6 miles) from Barcelona near the Cap de Creus Natural Park and a water park.
On travel websites it is said to offer "all comforts and a high quality service to anyone looking for a beach holiday in Costa Brava".
A Girona Police spokesman said: "At around 3.53pm, the mossos de la comissaria de Blanes (local police) received a note from the local police of Lloret de Mar that the bodies of two minors had been found at a hotel.
"When the officers arrived at the scene they found two children inside a room at a hotel.
"Girona Police are investigating the death of the two children.
"The police have detained the mother of the minors to find out what happened."
Lloret de Mar is a popular destination for British holidaymakers.
While the nightlife attracts an 18 to 30s crowd, the resort is also popular with families.