Hanover Lodge in Regent's Park, central London, is set to become the most expensive house in Britain when it is sold in the next few days for £120m.
An unnamed Ukrainian is set to complete the purchase from its owner, former conservative peer Lord Bagri, after contracts were exchanged in November.
It was originally thought he offer was between £100m and £120m, but the buyer confirmed an "impossible to refuse" offer of £120m was agreed after post-exchange problems were fixed.
The previous record for the most expensive house was £119m, paid for in 2011 by a Russian buyer for a Henley mansion.
Lord Bagri originally paid £5.9m to the Crown Estate in 1994 for a 150-year lease at Hanover Lodge. Bagri has since spent the last 10 years refurbishing the grade-II listed building.
Bagri's architect, Quinlan Terry, an apparent favourite of Prince Charles, was fined £25,000 in 2007 for allowing part of two lodges in the two-acre garden to be demolished.
The conversion of the 26,000-square-foot main house and a 7000-square-foot lodge is complete.
Permission has been granted planning permission to dig a large basement in the mansion. The Ukrainian buyer is also considering remodelling the interior of the three-story house, but no decision has been made yet.