Hundreds of thousands of landlords and their accountants are digesting the impact of George Osborne’s shock tax change unveiled in the summer Budget on July 8.

The tax increase, on which there was no consultation, will be phased in from 2017 and fully implemented by 2020.

The change was unexpected, and the new regime is highly complex, so investors and their tax advisers are only now fully grasping its effects. Many investors remain unaware of the change, or underestimate its severity.

All higher-rate taxpayers who own buy‑to‑let properties on which there is a large mortgage will pay substantially more tax. Some current basic-rate taxpayers will also be hit, because the change will push them into the higher-rate tax bracket.

Those who are worst affected will see:

● the actual tax they pay on their investment rising twofold or more;

● the tax rate payable rising above 100pc, meaning that more than all of their profit is paid in tax;

● a degree of tax that pushes them into loss, making their investment financially unviable and forcing them to increase rents sharply – or sell.

Scroll down for a worked explanation of the changes.

What is also becoming clear is that worst hit will be those modest, middle-class savers who have prudently chosen to invest in buy‑to‑let, often alongside pensions and Isas, as a means to supplement their income.