The Defence Secretary repeatedly refused to say the pledge was a “policy” and instead continued to call it an “aim” and an “ambition”.
Presenter Evan Davis asked the Tory MP: “On the immigration pledge, to get immigration down, have you costed that one?
“Has someone done some work and said ‘this is how much it will cost the exchequer’ because my understanding is the Office for Budget Responsibility thinks cutting migration does cost the exchequer. Have you guys costed that proposal?”
Fallon responded: “There’s been various academic work done on the cost of immigration.
“We’ve made it clear that we accept there is a cost and we want to make sure that British companies do contribute to the training of British workers when they want to fill that post.”
Davis said: “How much is it going to cost the exchequer to get immigration down by two thirds from its current level?
Fallon admitted: “We haven’t set out a formulation of how much it’ll reduce by each year, what we’ve set out is our ambition to continue to bear down on immigration.”
As Davis continued to press Fallon on whether it was Conservative policy to get immigration down to the tens of thousands, the Defence Secretary said: “It’s our ambition to get immigration down”
Davis pressed on: “What’s the difference between an ambition and a policy? You’ve had it in previous manifestos and you’ve palpably not delivered it... I assumed that by repeating it there was some meaning to it this time.”
Fallon: “Well it’s our aim to continue to bear down on immigration... It is an aim and we intend to continue to aim to aim to reduce the level of immigration as we’ve set out.”
Davis retorted: “It sounds like a pledge made in the morning has turned into a somewhat vague aim that doesn’t need costing by the afternoon.”
The car crash exchange did not go unnoticed, with political journalists comparing the interview with those featuring Diane Abbott last week.
Last week the shadow Home Secretary revealed a flaw in Labour’s costing over the party’s pledge to recruit 10,000 police officers to make up for the decline in numbers since 2010.
She told LBC’s Nick Ferrari this would cost £300,000 over four years, prompting Ferrari to ask: “What are you paying them?”
Abbott said she meant to say it would cost around £80m.
But Ferrari was still perplexed. He said: “If you divide £80m by 10,000 you get £8,000. Is that what you are going to pay these police men and women? Has this been thought through?”
The shadow home secretary insisted: “Of course it has been thought through.”
The figure was later corrected and Abbott told the BBC: “I do know my figures.”
She added she had “repeated them correctly in six other interviews”.
On Friday morning shadow chancellor John McDonnell said the Tories’ plan to dramatically reduce immigration to below 100,000 was “a joke”.
“They’ve said it three times, this is the third General Election campaign they’ve said it in,” McDonnell told BBC Radio 4’s Today programme.
The Labour MP added: “Michael Fallon last night was asked the question ‘have you costed your proposal, which has gone from a policy to an ambition’ he couldn’t tell us the costs of that.”