Halifax has found that the number of properties valued above the IHT threshold could be as high as 4 million in 2015 and 6 million in 2025, if house price inflation continues with no changes occurring over IHT.
In 1994, properties in only 5 constituencies were potentially liable to pay the tax.
The lender is sending a leaflet to around 600 opinion formers including all English and Welsh MPs, calling for the need to reform IHT.
So far, none of the major parties have committed to index linking IHT thresholds to future house price movements, but Halifax would like to see the current level of 263,000 increased to 390,000, index linking in the future.
Halifax group economist Tim Crawford says: Inheritance tax is a tax for the few that the many may now have to pay.