Sir Vince Cable has warned that the Tories have "adopted wholesale" policies from Ukip as he unveiled a campaign poster featuring Nigel Farage's face superimposed on to the Prime Minister's head.
The Liberal Democrat grandee, who is seeking to regain his former Twickenham seat, went on the attack as he dismissed Theresa May for her "ridiculous and damaging" immigration target and for "hand-holding" with Donald Trump.
His comments came as the Lib Dems put Brexit at the heart of their manifesto in a pitch to capitalise on unhappy Remain voters.
Speaking to party activists at a rugby club in Twickenham, Sir Vince said: "She has adopted wholesale the Ukip package of hard extreme Brexit taking us out of the single market ... with no preparation around science and the environment.
"He's (Nigel Farage) got every reason to be thrilled with what she is doing on Europe."
Sir Vince condemned the "regressive alliance" between Ukip and the Conservatives, where Ukip has withdrawn candidates to make way for pro-Brexit Tories.
He went on: "What's the point of having a Ukip candidate if the Conservative candidate is speaking for you?
"That's the issue we want to draw to your attention, the very closeness between the Conservatives and Ukip, particularly Mr Farage who has been such an inspiration to the Conservative leadership."
Sir Vince also weighed in on Tory plans to curb net migration to less than 100,000, saying Mrs May was isolated among ministers on wanting to keep the pledge made by David Cameron in 2010.
He told the Press Association: "The problem is that within the target, there are groups like students, who are not immigrants at all but were included in the number, and she has been absolutely implacable about keeping them out, even though they bring great benefit to the economy and they are not long-term immigrants.
"It's a perverse and foolish policy, which when we're in the coalition, she was in a minority of one trying to make it work."
The former business secretary is one of a number of senior Lib Dems hoping to win back their seats after the party suffered heavy losses in the 2015 election.
Asked how many seats the party could win, Sir Vince said: "We are not fixing a target, but we think there will be a substantial improvement."