Although 10 Downing Street - 3,800 sq ft of living space in a London SW1 address - is now valued at more than £4.5 million, its value has dropped by £462,000 since Gordon Brown moved in, claims the property website Zoopla.co.uk

The winner emerging tomorrow (May 6) - or later, if the first results are too close to call - will enjoy rent-free living which would otherwise cost £4,250 per week, it calculates.

Zoopla.co.uk, which provides a value for every home in the country, reckons that since Gordon Brown arrived at the address in June 2007 its value has fallen by 9.18% - costing the taxpayer more than £460,000 in value.

This is in stark contrast to his predecessor Tony Blair who saw its value climb from £1.65 million when he took office in 1997 to more than £5 million when he handed the keys over to Mr Brown.

If there is a change in leadership over the next few weeks, Gordon Brown could drop quite a few rungs on the property ladder as house prices in his own constituency of Kirkcaldy and Cowdenbeath are among the lowest in the land at an average of £120,910. This is compared with his current address in SW1 where average house prices are £920,361.

A victory on May 6, will represent a big move up for David Cameron and Nick Clegg as current prices in their own constituencies currently average £289,686 and £219,136 respectively.

Nick Leeming, commercial director of Zoopla.co.uk, says: "10 Downing Street is one of the most exclusive addresses in the country, with a valuation to match.

"It is one of the few properties that is never likely to come on the market and it takes millions of votes to secure the keys.

"Messrs Cameron and Clegg clearly have a struggle on their hands to wrestle those keys from Mr Brown who will be very reluctant to call in the movers."

Changes in value of 10 Downing Street under recent Prime Ministers

Gordon Brown (2007-present): Down from £5.03 million to £4.57 million - losing £165,000 per year.

Tony Blair (1997-2007): Up from £1.64 million to £5.0 million - gaining £335,910 per year.

John Major (1990-1997): Down from £1.8 million to £1.64 million - losing £24,600 per year.

Margaret Thatcher (1979-1990): Up from £692,000 to £1.8 million - gaining £96,700 per year.