Alan Farthing's comments come as the BBC tightens security in the wake of the death of one of its highest profile presenters.

Mr Farthing told the BBC, in his first public interview since Ms Dando's death: "I am devastated, and everyone around me is devastated.

"I cannot believe what has happened. I cannot understand what has happened.

"I cannot think for one moment what could go through someone's mind when they do such a thing to such a beautiful, caring, well-meaning and as perfect a person as Jill."

Crimewatch UK presenter Ms Dando, 37, left Mr Farthing's home in Chiswick, West London on Monday morning before she was killed with a single shot to the head on the doorstep of her home at 11am in Gowan Avenue, Fulham.

Consultant gynaecologist Mr Farthing, 36, had set aside this week to make final preparations for the couple's wedding in September.

He urged people who had any information to get in touch with the police.

He said: "My first emotions now are that whatever happens and whatever arrests are made it does not bring back Jill and that is desperately sad.

The BBC's Anna Ford: "Every sign that there is still much to do"

"Nevertheless, it helps me and Jill's family and it helps Jill's friends and colleagues to feel that we are doing everything we can to try and apprehend whoever is responsible."

Asked if Ms Dando ever discussed fears arising from her work on Crimewatch, or related to her celebrity, he said: "Not in any serious manner, not in any way that she was particularly scared.

The couple were due to marry in September

"There is nothing that has particularly run through my mind, and as you can imagine I have spent some hours, as have everybody who knows Jill, spent some hours desperately racking the back of their minds to see whether they can come up with any particular reason."

The couple met in 1997 on a blind date organised by a friend of Ms Dando's and got engaged in January this year.

After a string of relationships that buckled under the pressure of work, it seemed Ms Dando had finally found her perfect match.

In March last year the couple's growing love for each other was captured in a photo showing them sharing a kiss on a ski slope at the resort of Villeneuve in the French Alps.

Diamond ring

Less than a year later, after a New Year's trip to Australia, Ms Dando was seen wearing a diamond solitaire on her wedding finger.

Alan Farthing: "I cannot believe what has happened"

Then, on 31 January, the couple announced their engagement at a party for family and friends at the Tier restaurant, near Mr Farthing's home.

After the announcement a glowing Ms Dando said: "It is a very strong relationship. It's changed the way I look on life. When there are two of you, you have a lot more to share."

Mr Farthing, who is divorced and has no children, made the effort to spend as much time as possible with Ms Dando despite their busy careers.

Hitman clue

Ms Dando was killed by a man who shot her in the head at very close range with a 9mm semi-automatic pistol.

The execution-style manner of the shooting prompted speculation that she might have been the victim of an underworld hitman.

The officer leading the inquiry, Detective Chief Inspector Hamish Campbell, 42, said: "It could either be a stalker or a hitman. However, there are many theories to be explored and nothing will be left untouched."

The murder weapon was said by experts to be the weapon of choice of the professional criminal, and Mr Campbell appealed for information from anyone who might have been involved in the illicit supply of the firearm.

Wednesday's edition of Crimewatch File, for which Ms Dando was doing the voice-over, has been postponed following the murder.

Meanwhile the BBC has confirmed that Ms Dando's boss has been threatened by a phone caller. Tony Hall, the BBC's Head of News, was named by an anonymous caller who called the BBC's switchboard.