A UK aid worker held hostage after being kidnapped in Afghanistan has been killed during a rescue attempt, the Foreign Office has said.
Linda Norgrove, 36, from Lewis in the Western Isles of Scotland, was employed by US aid group DAI. She was seized with three local staff on 26 September.
Their two-car convoy was ambushed in the eastern province of Kunar.
Ms Norgrove was killed by her captors on Friday during a rescue mission by US forces.
Her colleagues were released unharmed last week.
The Briton is believed to have been taken by her captors from village to village as British, Afghan and other intelligence agencies worked in the remote and mountainous area of Kunar province to locate her.
Both the prime minister and Foreign Secretary William Hague were kept fully informed and British approval was given for a rescue mission to be mounted on Friday night, involving US forces with British officials offering advice.
In a statement, Mr Hague said the aid worker was "killed at the hands of her captors in the course of a rescue attempt".
He said: "Working with our allies we received information about where Linda was being held and we decided that, given the danger she was facing, her best chance of safe release was to act on that information.
"Responsibility for this tragic outcome rests squarely with the hostage takers.
"From the moment they took her, her life was under grave threat. Given who held her, and the danger she was in, we judged that Linda's best chance lay in attempting to rescue her."
Prime Minister David Cameron said: "My thoughts are with Linda's family, who will be devastated by this tragic news. She was doing valuable work for the Afghan people.
"Decisions on operations to free hostages are always difficult. But where a British life is in such danger, and where we and our allies can act, I believe it is right to try. I pay tribute to the courage and skill of all those involved in this effort, and join the foreign secretary in condemning hostage taking."
International Security Assistance Force Commander General David Petraeus said Afghan and coalition security forces did everything in their power to rescue Ms Norgrove.
He said: "Linda was a courageous person with a passion to improve the lives of Afghan people, and sadly she lost her life in their service. Our thoughts and prayers are with her family during this difficult time."
And Alex Salmond, Scotland's first minister, expressed his "deepest condolences" following her "extremely sad and upsetting" death.
"Ms Norgrove was a dedicated aid worker who was doing everything she could to help people in Afghanistan - hopefully that legacy of service in a humanitarian cause can be of some comfort to her loved ones in their time of grief," he said.
Ms Norgrove had been based in Jalalabad where she supervised US-funded reconstruction programmes in the eastern region of Afghanistan.
DAI president James Boomgard said the loss of a "beloved friend and respected colleague" was "devastating news" and sent his condolences to her family.
In a statement, he said: "We are saddened beyond words by the death of a wonderful woman whose sole purpose in Afghanistan was to do good, to help the Afghan people achieve a measure of prosperity and stability in their everyday lives as they set about rebuilding their country.
"Linda loved Afghanistan and cared deeply for its people, and she was deeply committed to her development mission. She was an inspiration to many of us here at DAI and she will be deeply missed."
A colleague who was working with her in Jalalabad told the BBC Ms Norgrove was a "very kind and hard-working lady".
They said: "You know she didn't need to go for the completion ceremony of that project, but the only reason she wanted to go there was to show her love and commitment to the people of Kunar province.
"Everyone in our office is crying including our Afghan and foreign colleagues. Starting from the boss of the office even to drivers and guards are shocked and crying. No one thinks that she was a colleague, but it looks like she was part of everyone's family and we have lost one of our dearest."
Ms Norgrove, who had travelled extensively, was an experienced aid worker who had been based in a number of countries.
She worked for the United Nations in Afghanistan and Laos and, prior to that, led a conservation and poverty reduction project in Peru.
The BBC's Bilal Sarwary in Kabul said the Dewagal valley, in eastern Kunar province, where she was held, is known for its difficult terrain. It is mountainous and densely forested. The valley is extremely remote.
There has never been any government control; it is virtually ruled by militants, tribal elders and powerful clans.
Various armed groups operate in the area, Afghans and foreigners can be targeted by gangs seeking ransom money, but they are sometimes sold on to militant groups.
DAI carries out aid work, often subcontracted by the United States Agency for International Development.
In July, a British private security guard was among four people killed in an attack on DAI offices in Kunduz, northern Afghanistan. Shaun Sexton, 29, from Northumberland, a former member of the Parachute Regiment, was working for the firm's security sub-contractor, Edinburgh International.
A month later, British doctor Karen Woo and nine other aid workers and translators were killed by gunmen, in the north-eastern province of Badakhshan, in what police said was a robbery.
Dr Woo worked for Christian charity the International Assistance Mission, providing eye care in remote villages.