Helen Johnston is a graduate of the London School of Hygiene and Tropical Medicine
Four aid workers kidnapped in northern Afghanistan last month have been freed by coalition forces, officials say.
British nutritionist Helen Johnston, Kenyan medic Moragwa Oirere and two Afghans were rescued in Badakhshan province on Friday after being captured on 22 May, the Nato-led Isaf said.
UK Prime Minister David Cameron called it an "extraordinarily brave, breathtaking" operation.
Five hostage takers were killed during the rescue, Nato said
The group - all employees of Switzerland-based aid group Medair - were making their way on horseback to the remote, mountainous province of Badakhshan, in north-eastern Afghanistan, when they were captured.
A fifth member - another Afghan national - was released soon afterwards.
David Cameron: "It was an extraordinary, brave, breathtaking operation"
According to Isaf, the others were found in a cave by the rescue team who arrived by helicopter under the cover of darkness.
Lal Mohammad Ahmadzai, spokesman for security forces in northern Afghanistan, said the hostages were in a "good condition".
Gen John R Allen, Isaf commander said the mission "exemplifies our collective and unwavering commitment to defeat the Taliban".
"I'm extremely grateful to the Afghan authorities and proud of the Isaf forces that planned, rehearsed, and successfully conducted this operation."
Mr Cameron said: "It was an extraordinarily brave, breathtaking even, operation that our troops had to carry out.
"I pay tribute to their skill and dedication."
The prime minister said the outcome should serve as a warning to those who chose to kidnap British citizens that they could expect "a swift and brutal end".
He said he had authorised the mission because he believed the risks to the workers' lives were getting greater - and since their release, had spoken to Ms Johnston herself, as well as her parents, Philip and Patricia and brother Peter.
"They are incredibly relieved about what has happened. It's just a huge joy that they are finally going to be reunited and they are all healthy and all well."
Mr Cameron said the rescue involved a number of British troops, none of whom were injured.
In a statement, the family of 28-year-old Ms Johnston said: "We are delighted and hugely relieved by the wonderful news that Helen and all her colleagues have been freed.
"We are deeply grateful to everyone involved in her rescue, to those who worked tirelessly on her behalf, and to family and friends for their love, prayers and support over the last 12 days.
"We greatly appreciate the restraint shown by the media since her abduction, and ask that they continue to respect our privacy at this special time."Criminal gang
Kenya's Ministry of Foreign Affairs thanked the British government, British High Commission in Nairobi, Medair and Isaf.
"The ministry further thanks the family of Miss Oirere and in particular Dr Oirere and his wife Milcah for remaining calm throughout this trying moment, and [for] the co-operation given to the government officials dealing with the matter," it said in a statement.
A UK Foreign Office spokesman said Ms Johnston and Ms Moragwa, 26, were receiving support from British Embassy staff in Kabul, while the two Afghan nationals were returning to their families in Badakhshan.
Police in Badakhshan, which borders Tajikistan, China and Pakistan, described the gunmen as part of a criminal group which was taking advantage of the challenging terrain and the loose grip that Afghan security forces had on the area.
But the BBC's Quentin Sommerville, in Kabul, says heavy machine guns, rocket-propelled grenades and AK-47s found during the raid suggest the kidnappers did have links to insurgent groups such as the Taliban.
Medair spokesman Aurelien Demaurex said the organisation was "relieved" its staff were safe and "immensely grateful" to all those involved in their rescue.
In 2010, a group of foreign medical workers, including six Americans, were killed in Badakhshan in an attack blamed on insurgents.