The new face of Ukip is a peddler of "hateful" ideas and a stooge of Nigel Farage, Lib Dem leader Tim Farron will say as he opens his party's conference in Brighton.
Mr Farron is set to dismiss incoming Ukip leader Diane James as being nothing more than a puppet of Mr Farage, as the Lib Dems look at imposing a special NHS tax.
He will tell delegates that Mr Farage remains the real focus of power in Ukip.
Mr Farron will say: "So, yes, Ukip have a new leader. It's the fourth new leader they've had in the last few years - and the last three new leaders were all the same bloke.
"So it's nice that they've got a new leader who is actually new. Well, I say 'new', 'new' in a 'Britain went to the dogs when we lost the empire' sort of way.
"But Ukip's policies are no laughing matter. And neither is Diane James. She/he leads a party whose policies are based on a vision for Britain that is closed, hateful and divided."
The attack will come as former health minister Norman Lamb announces plans for a panel of medical experts to consider the case for a special wage levy to fund the NHS and report back to the Lib Dems on their findings in six months' time.
Mr Lamb will say: "Let's look at the case for a dedicated health and care tax, shown on your pay packet. Let's look at whether we could use National Insurance as a basis for such a tax. We need to consider how we could make it more progressive. We need to think about fairness between generations.
"We must be honest with the British people. If we conclude that we all need to pay perhaps an extra penny in the pound then we must be prepared to say it."
Mr Farron is also set to use the gathering to ridicule the "three Brexiteer" ministers heading the UK's withdrawal from the EU.
He will say: "Liam Fox, the International Business Secretary. Has he read his job description, do you think? I'm going to guess that it doesn't include the line, 'The post holder should seek to undermine British business abroad and demoralise British business at home'.
"Of course, to be fair, Liam Fox did not undermine British business abroad or demoralise British business at home, because he could only have achieved those things... if anyone actually took him seriously. Still, he's getting a nice tan.
"David Davis, Brexit Secretary. Born-again liberal, against ID cards, detention without trial, snoopers' charter ... all the things that Theresa May is in favour of, incidentally. So that'll end well.
"But don't worry, we'll have taken back control. Which is a huge relief.
"And then there's Boris Johnson. Dispatched by the PM to go on tour to as many capitals as possible because she was concerned that, following the referendum, other countries might think Britain had completely lost its marbles.
"So she sent Boris Johnson to see them, so that they wouldn't think we'd lost our marbles, they'd now be absolutely certain."
The opening day of the conference will also consider liberalising vice laws and downgrading Britain's nuclear strike force.