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.

The killer shot Miss Dando on the doorstep of her home

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."

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."  Anyone with information can contact the murder incident room on 0181 246 0732 or Crimestoppers on 0800 555 111.

The Sun newspaper announces it is offering a £100,000 reward to help catch Ms Dando's killer. The reward brings the total on offer to £250,000.

Bullet clue in Dando murder

Detectives hunting the killer of BBC presenter Jill Dando say they may have an important new lead.  

Six distinctive marks were found on the cartridge case which have never been seen either in the UK or elsewhere, said Detective Chief Inspector Hamish Campbell.

The BBC's Robert Hall: "The killer may have left a trade mark"

Forensic experts in the UK, Europe and further afield have all inspected the tiny marks, made with hammer blows, but none recognised the symbols.

The marks would have had the effect of holding the bullet in place by "crimping" the cartridge case, but the firing of the bullet would not have been affected, said Mr Campbell.

He said the markings could have been a trademark, habit or idiosyncrasy.

The BBC's Graeme Mclagan: Mr Campbell said the unique marking may help inquiry

Mr Campbell appealed to gun dealers and anyone handling ammunition to get in touch if the markings meant anything to them.

He declined to say whether the revelation lent weight to one of his squad's two main theories - that Miss Dando, 37, was shot by an obsessive fan or a contract killer.

Bullet modification
An armourer source said the markings indicated that the bullet had been removed from its case, and was tampered with before being replaced. The source explained that modifications to bullets could be carried out by anyone with basic engineering skills or a working knowledge of guns, using information from a book on the subject.

 

Police are hoping the bullet will provide a long-awaited breakthrough

A bullet is usually removed from its case to make it into a dum-dum (which expands on impact and inflicts laceration for maximum injury), or to remove powder from the cartridge to make the explosion quieter.

The bullet would then be replaced, crimping the case to ensure it was held in place.

A particularly lethal version of a dum-dum involves drilling a hole in the top of the bullet, filling it with mercury, and then sealing it with wax. On firing, the mercury heats up inside the bullet and then expands, causing even greater injuries.

'Theory unlikely'

Brian Carter, the director of the Gun Trade Association, said he believed the killer was unlikely to have made the indentations to leave his own personal mark.

He said: "It sounds a bit too Graham Greeneish or James Bondish to me. I wonder whether someone took the bullet out to reduce the powder and reduce the noise but leaving enough powder in there to propel the bullet to kill someone."

A police source, not linked with the investigation, said the theory was unlikely if the killer had been loitering around Miss Dando's home for some time before the shooting.