A mystery woman spotted on CCTV in an off-licence could be a vital witness in the hunt for the killers of a student stabbed to death almost exactly five years ago today.

Samuel Guidera, 24, was knifed in the heart moments after leaving Penge East station on the evening of February 12, 2011.

Police have spent five desperate years investigating the killing. But despite a £20,000 reward, interviews with hundreds of witnesses, and the elimination of more than 1,000 leads, Samuel’s deadly attackers remain at large.

Today a senior Metropolitan police officer from the force’s homicide and serious crime command appealed for the woman pictured to come forward with any information that could aid the inquiry.

Pc Cliff Haines said: “She is important. She might be able to corroborate.

“She’s the very last [possible witness] we need to trace.

“She might have that nugget of information that means we can solve this case five years on.”

The image shows the woman visiting an off-licence on Lennard Road after getting off a bus, which police say would have passed the stop where Samuel was attacked at around the time of the incident.

The Greenwich University history and politics student had been on his way home when he was targeted. His killer or killers had stabbed him in the heart and taken his wallet.

He was found collapsed just before 10pm in Bailey Place near the junction with Newlands Park and rushed to hospital but pronounced dead less than an hour later.

Detectives believe Sam’s killers may have stopped him to ask to borrow his phone. Previous appeals have sought to uncover the story behind a mystery call made from his phone to an unknown number.

The number 07404 776433 was not stored in his phone and is not thought to belong to anyone he knows.

In the moments after that call was made, Sam misdialled “9999” for the emergency services.

Officers are also still pursuing this line of enquiry, and are keen to speak to anyone who can identify this number or who has a very similar one differing by just a few digits.

Samuel’s parents previously described the experience since their son’s death as a “life sentence from which we will never been released”.

"We still feel a sense of anger and all we can do is continue to cope with each day as it comes, with the help of medication, family, friends, whatever it takes," Sarah and Chris Guidera said in a statement

"We miss and think of him so much every single day, we cry, we love him so much.

“Time is not a healer, we will never get back to normal. The people we were before no longer exist.

“So please look at your son, brother, uncle, think how you would miss them and if you know anything come forward; don't let the cowards who took my son's life kill again."

Speaking to the Standard, Pc Haines also revealed a man held in connection with the investigation in 2011 was arrested because he had made a confession to a friend in an effort to “enhance his status”.

He was released without charge after police discovered his account was a “fantasy”, Pc Haines said.