An internal memo meant for MPs, accidentally sent to journalists yesterday, said the party was considering how higher earners could "make a further contribution" to the nation's finances.
Nick Clegg outlined similar ideas before the LibDem conference last year, calling for the top 10% - those earning more than £50,500 - to face higher taxes. His intervention led to talk of a tightening in pension tax relief.
Party sources said the inclusion of the line in the memo was a "mistake". They said they had no plans to set an arbitrary limit above which workers would be "clobbered", but added: "We don't retreat from what Nick said last year, which was that we believe we can have fair taxes that target the wealthy."
The call came as the leadership just defeated a vote calling for the reinstatement of the 50p tax rate and won a key debate backing its economic policy.
Today at the party conference in Glasgow Chief Secretary to the Treasury Danny Alexander will say increased taxes on the wealthy will be one of the party's key pledges in its next manifesto. And he will describe tax breaks for childcare - and unmarried couples - as a "fair way" to help families.
Business Secretary Vince Cable yesterday launched a scathing attack on the LibDems' Tory partners, accusing them of pedalling "ugly" politics and warning the City "spivs and gamblers" had returned.
Meanwhile, Michael Moore, the Scottish Secretary, at a fringe event last night placed Alex Salmond and the SNP alongside Nigel Farage's UKIP and hard right Tory Eurosceptics.
He said: "The First Minister has said nationhood rather than economics is at the heart of Scottish nationalism and Eurosceptics place sovereignty rather than jobs, trade or family finances at the centre of their own high-pitched case. But the arguments in favour of the UK family and the EU family are highly rational and very similar."