Police investigating the disappearance of Claudia Lawrence want to speak to a "mystery boyfriend" she spent the night with two days before she vanished.
The revelation comes a year after the chef disappeared on 18 March 2009.
She was last seen leaving work at the University of York. Her disappearance is being treated as suspected murder.
Her sister Ali Sims has urged anyone who was in a relationship with Claudia to come forward - even if it meant compromising a current relationship.
She made the plea while speaking on GMTV.
Talking about the mystery boyfriend, Det Supt Ray Galloway said: "Critically, information has come to us... whereby we understand that Claudia has told [someone] that she was actually out on one of the evenings, on one of the nights which immediately preceded her disappearance - out into the very early hours with what she described as a boyfriend.
We can't let people forget her - someone, somewhere knows something
Joan Lawrence, Claudia's mother
"And we are talking deep into the night, half past four or five in the morning.
"That is something we didn't know previously.
"I'm appealing [for that person to] please come forward because there is nothing suspicious in that, it is just the fact we do need to eliminate that person from our investigation."
In another development, police said Miss Lawrence was involved in a long-term, casual sexual relationship with another man when she vanished.
That man was not believed to be the man Miss Lawrence was with, nights before she went missing.
Miss Lawrence's father Peter said the new information about his daughter's relationships had come out "far too late".
He said: "It's unbelievable, in fact, that someone who has previously been interviewed by the police has suddenly remembered that he had a conversation with Claudia the day before she disappeared.
"First of all, that in itself is unbelievable, and secondly the fact that Claudia - who was always tired with her very physical job and with her early shifts and went to bed at 9.30 at night by the latest during the week - apparently said she was up until about 4.30 in the morning.
"On the other hand, it's another bit of information that's come forward and it has to be investigated."
North Yorkshire Police have released a video appeal to coincide with the anniversary which features on their website and on YouTube.
A new image of Miss Lawrence, at a family celebration in France in 2006, has also been released along with a redesigned poster with a picture which will be displayed on all police vehicles in the York area.
In a statement released through North Yorkshire Police, Miss Lawrence's mother, Joan, said: "I can't begin to describe the feelings that went through my mind when we received the call that Claudia had disappeared.
"I was numb at first, stunned, this sort of thing happens to other people."
She added: "We can't let people forget her - someone, somewhere knows something."
The hunt for Miss Lawrence is one of the largest investigations ever carried out by North Yorkshire Police, and has so far cost more than £600,000.
Detectives have travelled to Cyprus and the Republic of Ireland in the hope of tracing her.
The video features on North Yorkshire Police's website and YouTube
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