The public is being urged not to donate to a site asking for donations for the "Clauida Larence appeal".
North Yorkshire Police and the missing university chef's family both say they have no involvement with the site.
A police spokesman said: "It has been brought to our attention that a website going under the name of claudialawrence.com has been set up which is asking for donations to the 'Claudia Lawrence Appeal'.
"Please be aware that neither Claudia's family or North Yorkshire Police have had any involvement with this website.
"We strongly advise members of the public not to make donations.
"North Yorkshire Police is making inquiries regarding this website."
It is 12 days since 35-year-old Miss Lawrence was seen near her home on Heworth Road, York.
Det Supt Ray Galloway, the detective leading the search for Miss Lawrence, was due to appeal for information about her disappearance on the BBC1 Crimewatch programme at 9pm this evening.
Miss Lawrence is white, 5ft 6in, slim with brown hair and brown eyes. She was last seen wearing a white T-shirt, blue skinny jeans and white trainers. Poilce are seeking the small blue or green rucksack she used to carry her uniform to work at the University of York's Goodricke College.