Stephen Greenhalgh is the Deputy Mayor of London for Policing and Crime and formerly Conservative Leader of Hammersmith & Fulham Council

A cardinal principle of any successful government is first to do no harm – much in the same way that a good doctor cares for their patient. We all know that Labour’s instinct is for government to tax more and spend more.  But it is increasingly hard for London Labour MPs to sell their party’s national policy to introduce a new “Mansion Tax”. Yesterday a beleaguered Sadiq Khan tried his best at a Centre for London hustings on each party’s General Election offer to London.

We all know that this is not a tax on mansions but predominantly on the families of Londoners who own flats in zone 1, Victorian terraces in zone 2 and Edwardian semis in the suburbs.

Many of these may be asset rich but many are also income poor. They may be elderly people who bought their homes when they were affordable to them. They may even rely on London’s National Health Service now. But this tax policy will not benefit them. It is another tax on Londoners where the money raised would be spent outside the capital.

I bet that 98p in the pound will be raised on London property but no more than 15p of that pound – which is London’s proportion of the national population – raised by an incoming Labour government would be spent on London’s NHS. No wonder that an increasing number of London Labour MPs and council leaders are rejecting the idea including my old adversary, Stephen Cowan, who is the new H&F Labour council leader as well as Tessa Jowell – the Labour frontrunner to be the next Labour candidate for Mayor in 2016.

However, we should not forget that the “Mansion Tax” was originally the Lib Dem idea of Dr Vince Cable who is also a sitting London MP. Originally Vince wanted to get the Coalition to tax all homes worth more than a million pounds. Quickly the threshold was doubled to £2 million as Nick Clegg had just bought a house costing more than a million in Putney.

Early on in the life of Coalition Government I was sent on BBC Radio 4’s Today programme by Eric Pickles to squash the idea. Imagine my horror as a Conservative council leader to find myself debating a Conservative MP!

The “Mansion Tax” was originally a Lib Dem policy which has been adopted and adapted by Labour and is also supported by UKIP politicians. The Conservative MP on the radio at the time was Mark Reckless who is now the UKIP candidate for Rochester & Strood.