The region's daily newspaper, Ouest France, said yesterday that the information came from another building worker, who said that he recalled the man behaving suspiciously at the time.
He and his colleagues had considered approaching police soon after Caroline died during a school trip to the small town of Pleine Fougeres, near St Malo, in July 1996. They decided against coming forward at the time, but the suspicions of at least one of the builders were re-ignited when he saw the sketch of a man suspected of committing the crime.
Caroline's father, John Dickinson, travelled to France last night to discuss the new development in the case with the examining magistrate leading the inquiry, Renaud Van Ruymbeke. Before leaving Britain, Mr Dickinson, said: "I can confirm that the person they are looking for is somebody who was seen in Pleine Fougeres, near the youth hostel, who may or may not have been working on a building site or road improvement scheme at the time of the murder."
The 10 gendarmerie officers still working on the case were said to be taking the anonymous tip-off "very seriously." They were checking with a number of building companies who were carrying out work in the town at that time to try to match the artist's sketch with the name of an employee or former employee
Caroline was attacked while she slept in a dormitory with four other schoolgirls during a trip with the Launceston Community College in Cornwall.
The investigation, frequently criticised in the British press, has involved the pursuit of several false leads over the past three years. A drifter aged 39, who was arrested soon after the murder, and confessed under gendarmerie questioning, was cleared by DNA evidence. And the local magistrate in charge of the case was removed in response to complaints by the Caroline's family and the press.
Genetic tests were also carried out on all adult male inhabitants of Pleine Fougeres and all men on the school trip - 2,000 people in all. No matches were found.
A dozen people resembling the police image of the killer - assembled from witness accounts of a man seen near the scene - have been interviewed and cleared by DNA tests.
The police image is of a thick-set, white man in his early 30s, with a broad nose, black hair and prominent eyebrows and sideburns.