All other major Budget measures were accompanied on Budget day by an impact assessment called a Tax Information and Impact Note (TIIN).

Government sources have claimed that the National Insurance assessment will be published when the full legislation is prepared next year. However, the assessment was quickly announced after The Telegraph reported that the assessment had been postponed.

TIINs assess the likely impact of a policy change on the economy, families, individuals and those with protected characteristics under the Equality Act.

The government’s National Insurance review said: “Secondary Class 1 NICs are obtained from employers rather than individuals. Therefore, there is no direct equality effect. “It is not possible to assess whether there will be different impacts on groups that share protected characteristics,” he said.

In response, the EHRC wrote: “As the regulator of the Public Sector Equality Duty, it is important that we remind public bodies, including government departments, of their legal obligation to consider the potential impacts on equality of the decisions they make.”

signed letter Baroness Falknerwho chairs the EHRC, stressed the importance of a full impact assessment in case the policy change is challenged later.