THE father of missing chef Claudia Lawrence described today how ''the strain is intolerable and the sorrow unbearable'' as he marked 50 days since his daughter was last seen.

Peter Lawrence made a direct appeal to whoever has taken his daughter, originally from Malton, North Yorkshire, asking whoever it is to search their conscience.

Miss Lawrence, 35, has not been seen since March 18 when she was last spotted near her home, where she lived by herself in the Heworth area of York.

She never turned up to her job in a York University kitchen the next day and, since then, North Yorkshire Police has launched one of its biggest ever investigations to find her.

Today, North Yorkshire solicitor Mr Lawrence, 62, said: ''It is now 50 days that Claudia has been away from us.

''The longer it goes on, the worse it gets for myself, the family and her friends. The strain is intolerable and the sorrow unbearable.

''I want to make yet another appeal to whoever is responsible for taking her away from her life in York to come forward so that she can be reunited with all of us who love her dearly.

''You know who you are. Search your conscience. We want Claudia back.''

Earlier this week, Mr Lawrence said: ''I very much do believe that Claudia is still alive.''

Yesterday, detectives leading the inquiry said they had new information about two men they want to trace.

They said new information had come to light about two men spotted at Miss Lawrence's front door in York a week before she went missing in mid-March.

Officers have already said they want to trace two men seen outside the front of Miss Lawrence's home on Heworth Road.

Yesterday, a spokesman said another witness has come forward and detectives have now pinpointed the sighting to March 10. They also now believe both the men were Asian.

He said: ''One of the men was looking towards the downstairs window, even though the curtains are always drawn, whilst the second male was looking up at the first floor window.''

The spokesman said one of the men is described as 5ft 5ins to 5ft 6ins tall, with a distinctive, long, thin face with a pointy nose and dark circles under his eyes. He has a long, jutting jaw and dark, straight hair with a fringe and was in his 20s to early 30s.

The man was wearing what appeared to be a heavy coat even though it was a hot day.

The second male was of a heavier build, 5ft 7ins to 5ft 8ins tall, wearing a waist length jacket and jeans that had rivets around the back pockets.

The spokesman said: ''Police are appealing for anyone who saw these men in the area of Claudia's house or for the men themselves to come forward as they may have been at the house for an entirely innocent reason.''

He said there have now been more than 1,000 calls to the incident room, around 1,270 properties have been visually searched and the investigation has generated 1,096 reports and statements have been generated.

North Yorkshire Police have now formally classified the investigation into her disappearance as one of suspected murder.

Detectives said they still have not identified any of the other people they want to speak to in connection with the inquiry.

These include a man and woman seen on Melrosegate Bridge, in York, at around 5.35am on Thursday, March 19 - the day after Miss Lawrence was last seen - and a couple seen arguing on University Road on the same morning.