The think tank said the measure could raise billions of pounds for the Treasury while addressing what it describes as preferential tax treatment for property investors.

The NEF has proposed the reform as part of a broader package of tax changes it believes could make the system fairer and generate additional public revenue.

In a recent report, the organisation estimated that applying National Insurance to rental income could raise as much as £3.2 billion annually.

Under the current system, landlords pay income tax on profits from rental properties but are generally exempt from National Insurance contributions because rental income is classified as investment income rather than earnings.

While landlords may choose to make voluntary NICs to protect their entitlement to the state pension, there is no requirement for them to contribute in the same way as employees or the self-employed.

The NEF argues that this distinction creates an unfair advantage for landlords compared with workers whose earnings are subject to National Insurance.

George Bangham, head of social policy at the NEF, said: “When landlords make money from rental income, they are not asked to contribute the same as everyone else.

“This is clearly unfair, income from renting out a property should be treated the same as income from work.”

The NEF described the current exemption as an “unwarranted tax break” and said extending National Insurance to rental income would help create greater consistency across different forms of income.

To mitigate the impact on landlords, the think tank suggested that any changes could be accompanied by the reintroduction of mortgage interest relief, abolished by former chancellor George Osborne.