Speaking in public for the first time since Monday's killing, Mr Farthing, a consultant gynaecologist who was due to marry Ms Dando, 37, in September, said: "I am devastated, everybody around me is devastated ... . I cannot believe what has happened, I cannot understand what has happened. I cannot think for one moment what can go through someone's mind when they do such a thing to such a beautiful, caring and perfect person as Jill."
Mr Farthing said he had last spoken to her at 7.25am on Monday when he called her at his flat in Chiswick while at work at St Mary's Hospital in Paddington, west London. Just a few hours later, rushing off to a clinic, he received a pager message from her agent, saying he should get in touch.
He was asked whether he blamed Crimewatch, the programme Ms Dando presented, or her work at the BBC for her killing. He responded: "I don't know is the honest answer, and I don't know that the police have any more information. There is somebody who does have that information and the sooner they impart that to the police the better."
Asked whether he was concerned about the potential risk of retaliation from someone who had been convicted through the programme, he said: "Not specifically. It was more worrying, on the occasions when she decided to drive herself to and from Crimewatch, that she had to go into the BBC car park at one o'clock in the morning to drive herself home."
Mr Farthing said Ms Dando had discussed with the BBC the prospect of danger in presenting the show before she took the job. "Jill was concerned to find out whether there was any previous threat. She was reassured strongly. I'm not aware that there has ever been any threat in relation to the programme subsequently."
Mr Farthing was asked whether he was concerned that the murder may remain unresolved. He said: "My first emotions are that whatever happens and whatever arrests are made it does not bring back Jill, and that is desperately sad. Nevertheless, it helps me and it helps Jill's friends and colleagues to feel that we are doing everything we can to try and apprehend whoever was responsible.
"Somebody has planned this and somebody around that person will have noticed a change in behaviour, I would imagine, and somebody has suspicions that this person may have done this dreadful act. And if there is any thought in that person's head to get in contact with the authorities, I would appeal to them please not to think any further about it but to go ahead and give the police any information they can."
Detectives are investigating whether Ms Dando's killer tricked his way into the television presenter's home weeks before shooting her dead. It emerged yesterday that Ms Dando's house in Fulham, west London, had recently been sold and that dozens of people had probably been shown around the property in recent months.
The police will investigate whether the killer pretended to be a house buyer to gain information about his target and help to plan the murder. Detectives are to examine the estate agent's files and are expected to question some of those who viewed the house.
Last night's edition of the television programme Crimewatch File, for which Ms Dando had done the voiceover, was postponed.