Jo Swinson has “categorically” ruled out helping to make Jeremy Corbyn prime minister in the event of a hung parliament.
The Lib Dem leader made the commitment on Tuesday morning, accusing the Labour leader of being “not fit for the job of prime minister”.
Swinson’s comments mean she would not only refuse a formal coalition, but would not prop up a Corbyn-led Labour government by offering it a confidence and supply deal.
If neither the Conservatives or Labour secure a majority on December 12, the Lib Dems could once again become kingmakers.
But speaking at her party’s campaign launch in Westminster, Swinson said: “I am absolutely, categorically ruling out Lib Dem votes putting Jeremy. Corbyn into No.10.”
Swinson said her party must be “ambitious” enough to aim for Downing Street itself.
“What I am offering to the public is a Lib Dem government,” she said. “A Lib Dem government will stop Brexit.”
But contrast, SNP leader Nicola Sturgeon appeared open to the idea of striking a deal with Labour to put Corbyn in No.10.
The Scottish first minister told Sky News this morning she would “drive a hard bargain” in any talks.
“Let’s see what the arithmetic is. There will be no formal coalition, it would be issue by issue,” she said.
The Lib Dems have pledged to revoke Article 50 should they win power, in an attempt to hoover up both Tory and Labour pro-Remain voters.
Swinson said of Corbyn: “On the biggest issue of the day, he has prevaricated and will not give a straight answer.
“Even now if you ask him whether he is Remain or Leave he will not tell you how he would vote.
She added: “His plans for the economy would take us back to the 1970s. I believe he would be a threat to our national security.”
Speaking in Harlow at the same time as Swinson, Corbyn said Labour would “get Brexit sorted within six months”.
The Labour leader has proposed negotiating a new deal with the EU and then putting that to a referendum alongside the choice to Remain.
Corbyn insisted Labour’s plan for Brexit was “clear and simple” and “really isn’t very complicated”.
“It won’t be a rerun of 2016. This time the choice will be between leaving with a sensible deal or remaining in the European Union.