"The opposite is true, because it will affect landlords who have big mortgages. Very wealthy landlords who don't have to borrow won't be touched.

"The Government has also omitted to say is that it will move thousands of landlords from lower rate to higher rate, and some into additional rate."

The latter anomaly arises, Mr Cooper explains, because mortgage interest, required to be paid by landlords to their bank, will effectively be viewed by HMRC as income - on which tax is then due.

Thus some landlords will be taxed where they make losses. In its letter to HMRC, Omnia states that "the measure will result in some landlords being liable to tax on an economic loss due to a theoretical, if not actual, increase in income".

Mr Cooper believes mortgaged landlords will be forced to increase rents to cover the higher tax, or evict tenants in order to sell.

The Government has 14 days in which to respond, after which the matter moves to a judicial review - the formal process in which a court rules on whether or not legislation is valid.

Cherie Blair and her property fortune

While commentators have quickly siezed on the irony of a former Labour leader's wife taking a Conservative government to task over its attempts to increase property tax, they have also highlighted out Mrs Blair's personal interests in the issue.