The plan, unveiled by Prime Minister Boris Johnson last week, seeks to act as a deterrent to migrants who make illegal boat crossings to Britain from France. It has drawn heavy criticism from political opponents and campaigners.

In an exchange of letters with Patel, the top official in the Home Office highlighted uncertainty over the scheme's value to the taxpayer.

The government has said it would contribute an initial 120 million pounds ($156 million) to the scheme.

"I do not believe sufficient evidence can be obtained to demonstrate that the policy will have a deterrent effect significant enough to make the policy value for money," Home Office Permanent Secretary Matthew Rycroft said.

Patel ]acknowledged the concerns, but stated her belief that without taking action to stop the crossings, both the monetary costs and the loss of life among those who attempt to navigate the busy shipping channel would rise.

"It would therefore be imprudent in my view, as Home Secretary, to allow the absence of quantifiable and dynamic modelling ... to delay delivery of a policy that we believe will reduce illegal migration, save lives, and ultimately break the business model of the smuggling gangs," she wrote.

Last year, more than 28,000 migrants and refugees made the crossing from mainland Europe to Britain, a fraction of the number arriving in other European countries, but enough to keep immigration a politically sensitive topic among some voters.

Concerns over immigration were a big factor in the 2016 Brexit vote, and Johnson has been under pressure to deliver on his promise to "take back control" of Britain's borders.

($1 = 0.7676 pounds)