Reform UK's Zia Yusuf warned Rachel Reeves against slashing the cash Isa allowance. The Chancellor is reportedly poised to announce plans to limit the £20,000 annual tax-free cap.

But Mr Yusuf, who heads up Reform's Department of Government Efficiency (Doge), wrote on X: "Unbelievably stupid from Reeves. Punish the segment of society who are actually saving so they won’t be a leech on the state. She is the worst Chancellor of all time."

The Chancellor is expected to announce plans to reduce the cash Isa allowance in her Mansion House speech next month.

Ms Reeves is said to believe the move, reported by the Financial Times, will encourage savers to shift from cash to shares to boost Britain's economy.

It would be the biggest overhaul of Isas since they were introduced in 1999.

Ms Reeves confirmed last month that she had no plans to reduce the total amount that can be paid into Isa products each year.

But she did not rule out cutting the allowance for cash Isas.

She said: “I do want people to get better returns on their savings, whether that’s in a pension or in their day-to-day savings”.

A Treasury official confirmed that the Government was “looking at options for reforms to Isas that get the balance right between cash and equities”.