As many as seven in ten migrants in Calais may be reaching the UK, police figures suggest.
Research by French authorities estimates that 70% of those who are "processed" in the area of the port leave the vicinity within a four-month period.
Kent Police chief constable Alan Pughsley, who forwarded the data on to the parliamentary Home Affairs Select Committee, said: "They cannot ascertain whether these migrants leave to go elsewhere in France, or whether they enter the UK.
"Either way, their figures identify a transient migrant population."
Mr Pughsley was asked, after appearing before the committee last month, to provide written evidence on the number of migrants repeatedly attempting to get to Britain.
He said: "The exact figure is difficult to establish due to the migrants using fake identities or giving false details to Border staff.
"The figures provided by the police and partner agencies can only account for migrants that are actually found during relevant searches. The actual figure of migrants entering the country therefore may be a lot higher."
The disclosures come as the Government struggles to get a grip on the biggest crisis since the Tories' election victory.