The former Tory leader said that Richard Tice's party will "simply strip votes away" from the Conservatives - and give Labour a majority government able to "bulldoze" laws through Parliament.
Sir Keir Starmer wants to build a closer trading relationship with the EU if he wins the election.
Sir Iain went on: "As someone who voted against the Maastricht Treaty in the 1990s, and against every single additional treaty following that, and as someone who was deeply involved in the Brexit campaign, I have of course sympathy for those who are frustrated that the pace of change has not happened as they had hoped.
"Whilst some significant things have been delivered, there is yet much more to be done. There is, of course, still more to be done to take advantage of the flexibility and independence that Brexit provides.
"Despite that frustration, we need to look at the reality of what could happen at the next election.
"Any vote for Reform is taking away only from the Tories. If you want to re-enter the European Union and if you want to align closer to Europe as Labour plans to do - if you want to have an unsustainable rush to net zero with extra taxes and regulations, then if you vote Reform, that is exactly what you will get." Sir Iain continued: "You will get a Labour government with a large majority that will be able to bulldoze anything they want through.
"All of the things that anyone who votes for Reform stands for will be lost. The only hope they've got is a Conservative Party that gets on and does what is necessary. To do that, they need another mandate."
Defence Secretary Grant Shapps insisted that "a vote for Reform is a vote for Labour" and risks "irrevocable damage" to Britain.
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