The reward from The Sun brings the total on offer to £250,000. The rest has been pledged by the Daily Mail newspaper and Crimestoppers.

Stephen Cape : "No clear motive for the killing"

Miss Dando, who presented Holiday and the Six O'Clock News as well as Crimewatch, was shot dead outside her home in Gowan Avenue, Fulham, south-west London, on 26 April.

Despite a huge police operation and numerous appeals, no-one has yet been charged.

A Sun spokesman said: "By putting up this reward our hope is to provide the police with a vital clue that will finally catch Jill's killer.

"The amount of money on offer is now so big that we hope it will trigger someone who has been holding back a crucial piece of information to come forward."

The killer shot Miss Dando on the doorstep of her home

But while the case has been given wide support and coverage by the media, the BBC's Stephen Cape told BBC One's Breakfast Time that police felt speculation surrounding it was "certainly unhelpful".

Recent rumours reported in the press that a dum-dum bullet or a gun silencer were used have been scotched by police.

"The latest piece of speculation that police are not happy about - that the Russian mafia are involved in some way - as far as the police are concerned that's not true either," said Mr Cape.

But the markings on the casing of the bullet that killed Ms Dando are "very important", he said.

The markings were found only last week, and show that someone deliberately altered the bullet to reduce the sound of the shot, he said.

Jon Roseman, Miss Dando's agent for 10 years, said he was optimistic that Miss Dando's brutal killing would be solved, and he urged people to rack their brains for new leads.

'Difficult to believe'

"Everybody who was close to Jill remains confident that this dreadful crime can be solved," insisted Mr Roseman.

"But the inquiry team need all the help they can get from the public.

"Anyone who was in the vicinity on the day of Jill's murder and might have information and has yet to come forward - or indeed anyone who thinks they may have any information about the killer - should contact the police."

Mr Roseman, who runs the London-based Roseman Organisation agency, said he and his colleagues were still struggling to come to terms with the murder.

'So vibrant'

"Because Jill was so vibrant, I still find it very difficult to believe. What happened is so surreal that it's very difficult to get any kind of 'closure'.

"I don't think I will be able to really grieve until they catch her killer.

"I need a reason for her murder, and until I understand why someone wanted her dead then I don't think I will ever fully accept it."