Deputy Labour leader Harriet Harman has apologised for branding Chief Treasury Secretary Danny Alexander a "ginger rodent".
Ms Harman - a former equalities minister - made the comments at the Scottish Labour conference in Oban.
A spokeswoman said later that Ms Harman admitted the comments were wrong and she had now "apologised personally by telephone to Danny Alexander".
Mr Alexander wrote on Twitter that he was "proud to be ginger".
The Lib Dem MP for Inverness, Nairn, Badenoch and Strathspey added that "rodents do valuable work cleaning up mess others leave behind".
In her speech, Ms Harman said many people who voted Lib Dem in May "believed that they were a progressive anti-Tory party".
She said they "woke up" after the coalition deal to see Lib Dem leader Nick Clegg with Tory prime minister David Cameron in the rose garden of Number 10 Downing Street.
The deputy Labour leader said there was "incredulity" at seeing Mr Alexander, a Scottish MP, becoming "the front-man for the Tory cuts".
"Now, many of us in the Labour Party are conservationists - and we all love the red squirrel," Ms Harman said.
"But there is one ginger rodent which we never want to see again - Danny Alexander."
Ms Harman went on to attack Mr Alexander's other party colleagues north of the border, telling delegates: "There's something deeply unnatural that's happened in Scotland.
"Without asking anyone in Scotland, the government has been carrying out a programme of genetic modification - political genetic modification.
"This mutation has contaminated every Lib Dem councillor, it's affected every Lib Dem MP and Lib Dem MSP.
"They've all mutated into something alien to Scotland - Tories."
Ms Harman said: "There's only one thing left to do - these political mutants must be got rid of next May at the ballot box."
She insisted Labour had learned the lessons of its general election defeat.
Ms Harman said the Scottish Parliament election will send a signal to the UK government - "it is the beginning of the end".
Mr Alexander later wrote on Twitter: "Red squirrel deserves to survive, unlike Labour."
Scottish Liberal Democrat election chairman, George Lyon, said: "There are no depths to which the Labour Party will not stoop.
"They aren't fit to be in opposition, let alone in government."