Three British friends have died in a "horrific" car accident in South Africa.
Miranda Harris, 66, Chris Naylor, 58, and Mr Naylor's wife Susanna, 54, died in the crash in Port Elizabeth on Monday, the Christian charity A Rocha International confirmed.
Ms Harris's husband, Peter, 67, was being treated in hospital.
Mr and Mrs Harris's home is in West Lavington, Wiltshire; while the Naylors lived near Burford, Oxfordshire.
The charity said the deaths came "as a profound shock to everyone".
"Above all, their families and friends are held in our prayers," it said.
The couples had travelled to South Africa to work and look for "possibilities for partnership in wildlife conservation", the charity said.
The Rev Dave Bookless, director of theology at A Rocha International, said the deaths "leave a huge and irreplaceable hole in our lives and our organisation".
Dr Bookless said: "Each one of them was a remarkable and inspiring individual, and the loss of all three is incalculable.
"We have been overwhelmed with prayers, condolences and offers of practical support from every corner of the world."
Children at bedside
Dr Bookless said Mr Harris was "still sedated but stable" in hospital in Port Elizabeth.
He said that two of his children and others from the A Rocha family were at his bedside.
A driver of the vehicle in which the four friends were travelling was also being treated in hospital.
The car reportedly came off the Swartkops Bridge after it collided with another vehicle on the N2 motorway in Bluewater Bay.
A Rocha International, which engages communities in nature conservation, said Mrs Harris had co-founded the charity with her husband after they had run its first field study centre and bird observatory in Portugal in 1983.
Mr Naylor, a father-of-three, had been executive director since 2010.
Susanna Naylor was a science teacher at Christ Church Cathedral School, Oxford.
The Naylors had been involved with the charity since they co-founded A Rocha Lebanon in 1996, after previously working as teachers in the Middle East.
The Foreign Office said it was supporting the couples' families.