The photograph, which was published on Twitter, shows a number of bullet points in a document held by Nick Clegg’s key strategist, Ryan Coetzee.
One of the points shows the wording “introducing a banded High Value Property (the ‘mansion tax’) [on] residential properties worth over £2m”.
This puts the Lib Dems further at odds with their coalition partners, who have persistently criticised shadow chancellor Ed Balls’ pledge to introduce a mansion tax should Labour gain power in 2015.
Earlier this year, the Lib Dem leadership appeared to backtrack on a mansion tax, with Danny Alexander, chief secretary to the Treasury, allegedly preferring to opt for a higher band of council tax.
However the Lib Dem pre-manifesto, which was voted on at the party’s 2014 conference, briefly outlined proposals for a mansion tax, although not in the same detail as their Labour counterparts.
The Conservatives have described a mansion tax as “unworkable”, with exchequer secretary to the Treasury Priti Patel saying a “homes tax won’t raise the money [Labour] says it will”.
A Lib Dem spokesperson declined to comment on any of the bullet points, and denied that the document had been deliberately leaked.