Richard Whelan was travelling home on the top deck of a London bus when a man in a hooded sweatshirt launched the vicious assault.

The victim’s girlfriend tried to fend off the attacker but could not prevent the fatal stabbing. After the attack Mr Whelan’s assailant calmly walked off the No 43 bus in Holloway Road, North London, leaving his victim bleeding to death.

Mr Whelan, 28, was taken to the nearby Whittington Hospital, where he was pronounced dead an hour later.

Before the assault Mr Whelan and his girlfriend had spent the evening having a few drinks on Upper Street in Islington. Detectives said that the murderer boarded the bus at the same stop as the couple at 9.45pm on Friday. All three climbed the stairs to the top deck and the couple settled down for the ride home. Within minutes, however, the man began throwing his chips at other passengers.

A number of the people on the upper deck moved downstairs but Mr Whelan decided to talk to the man, which ended in the attack. In a desperate attempt to save her boyfriend’s life, Mr Whelan’s girlfriend helped him down the stairs before he collapsed on the lower deck.

Some of the passengers on the bus came to Mr Whelan’s aid before paramedics arrived.

A post-mortem examination will be carried out today.

Detective Chief Inspector John Macdonald, who is leading the murder investigation, said that Mr Whelan had behaved reasonably throughout the altercation with his assailant. “This was a totally unprovoked attack by someone who obviously carries a knife as a matter of course,” he said. “Anybody could have been the victim of this crime. The victim had done nothing wrong at all.

“It’s the casual way in which the suspect acted and the level of violence used against someone who was enjoying a night out with his girlfriend that makes it all the more important that we identify and arrest this man.”

Mr Macdonald said that Mr Whelan’s girlfriend, who had been with her partner for a couple of years, was “in a terrible state of shock” and was not yet able to relive the events in any detail.

Mr Whelan’s family are also struggling to come to terms with their loss. “The large close-knit family are devastated by this,” Mr Macdonald said.

Mr Whelan’s mother died some years ago but the victim is thought to have a number of sisters and lived at home with his father in Kentish Town, North London.

Several witnesses have come forward but police are asking for anyone else who was on the bus when Mr Whelan was stabbed to contact them. They also want to speak to anyone who was travelling on a second No 43 bus behind when the attack took place.

The knifeman is described as black, in his 20s, just under 6ft, with afro-style hair — perhaps two to three inches in length. He was wearing a dark jacket, possibly leather or a puffa coat. When he boarded the bus he was wearing a hooded top with the hood up.

Anyone with information about the attack can contact the incident room on 020-8345 3985 or Crimestoppers on 0800 555111.

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