As many as 150 people are feared dead after a passenger plane crashed in the French Alps after sending out a distress signal.
Many of those on board the Airbus A320 operated by low-fare airline Germanwings are thought to be German, with more than 40 Spaniards also likely to have caught the flight.
Prime Minister David Cameron and Deputy Prime Minister Nick Clegg said their thoughts were with the families of the passengers.
Germanwings, which is owned by German carrier Lufthansa, confirmed that the flight, carrying 144 passengers and six crew, crashed on its way from Barcelona to Dusseldorf.
The budget airline said its thoughts and prayers were with relatives of the passengers and crew. French ministers were on their way to the crash site - a remote spot near the town of Digne.
Among them was French environment minister Segolene Royal, who was in London for a meeting with the Prince of Wales, which she cancelled to head back to France.
A room was opened in Barcelona's Terminal 2 to attend to relatives of those who were on the flight.
The plane that crashed was called "Mannheim" and made its first test flight in November 1990.
It was then purchased by Lufthansa and had its first commercial flight in February 1991.
Experts said the aircraft would have been nearing the end of its commercial life after more than 24 years of service.
Spain's King Philip VI and Queen Letizia officially cancelled their state visit to France after they were told about the crash by French Prime Minister Manuel Valls.
Lufthansa and Germanwings have set up a free telephone hotline - 0800 11 33 55 77.
Mr Cameron's official spokesman confirmed that the Prime Minister had been told of the crash.
The spokesman went on: "He has been informed of the tragic news of the aircraft that has been lost over southern France and he would wish to express how his thoughts are very much with the families and friends of all of those who were on board that flight."
He added that the Foreign Office was making inquiries about whether any British nationals were on board, but had no information to release at this stage.
He said UK air accident investigators would offer any assistance requested by the French or German authorities.
Mr Clegg said: "My heart goes out to the family and friends of those people who appear to have lost their lives in this devastating air crash in the Alps."
French Interior Ministry spokesman Pierre-Henry Brandet said debris had been located and that the plane had crashed at around 6,500ft (1,980m) up in the Alps.
Mr Brandet said he expected "an extremely long and extremely difficult" search and rescue operation because of the area's remoteness.
He added that the plane had sent out a distress signal at 10.45am.
France's transport minister, Alain Vidalies, said the Airbus had sent out a distress signal at a "worrying" altitude before crashing.
He went on: "This plane then crashed near the Prads commune. It crashed in a place that is inaccessible by road. The overview from the helicopter simply permitted a view of the wreckage and a few bodies around the plane."
It is thought that the Germans on board were tourists returning from Barcelona and Palma in Majorca.
Some 45 passengers had Spanish surnames, according to Spanish government sources. The Spanish government said it had set up a crisis Cabinet to deal with the crash.
Spain's Prime Minister, Mariano Rajoy, told a press conference: "I have had the opportunity to have a phone conversation with the German Chancellor (Angela Merkel). We have exchanged opinions about this very unfortunate and very sad accident.
"I have spoken to the King (of Spain), who is in France, and I have spoken with the head of the Catalan regional government.
"All I can tell you is that I am very sad about this very dramatic accident. We are going to do everything in our hands to help the families and give them our support."
Earlier, French President Francois Hollande has said it was unlikely anyone had survived the crash. He had spoken with Chancellor Merkel and was also due to speak with the Spanish king.
In a joint press conference, Mr Hollande and King Philip said that those on the plane were Spanish, German and Turkish.
Mr Hollande said: "We will have to work to find out all the causes of what has happened and will work closely with the relevant authorities both in Spain and in Germany, and with the victims' families.
"It is a test we will pass because it is a tragedy which has occurred on our soil, and we will look to see if there are other consequences of the accident, which took place in a particularly difficult area to access.
"I don't know yet if there are any inhabited areas there which were affected. We will find out in the coming hours.
"We have solidarity in our minds as our utmost thoughts."
King Philip, who was on his first state visit since acceding to the throne in June last year, said at the press conference in Paris: "We know there are many victims, we don't know if there are any survivors.
"Amongst the victims there are a high number of Spanish nationals alongside Germans and also Turks.
"In the first place I would like to express on behalf of the Queen and President Hollande and myself my profound condolences from the bottom of my heart to the families who have lost loved ones.
"I would like to transmit our pain to the German and Turkish authorities for their dead and I would also like to express to the French authorities, the French president and the prime minister our gratitude for all the measures they have immediately put in action.
"The operation to attend this catastrophe is very complicated because of the location where it took place.
"After conversations with President Hollande and Spanish Prime Minister Rajoy, we have decided to cancel our visit and hope to rearrange it at a later date."
Mr Cameron later tweeted: "Today's plane crash in the Alps is heartbreaking news. The UK will do everything it can to support the French emergency services."
Mrs Merkel said she would travel to the crash site tomorrow.
The German leader said she was "shocked" by extent of the crash, "which will be investigated very carefully".
"Our thoughts are with the victims and their loved ones," she added.
Airbus said a team of technical advisers had been sent to the scene to help the French air accident investigators.
It added: "Airbus will make further factual information available as soon as the details have been confirmed and cleared by the authorities for release.
"The concerns and sympathy of the Airbus employees go to the families, friends and loved ones affected by the accident of Flight 4U9525."
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