Research by investment service Wealth Club found that 96.1% of investors believe taxes will increase over the next 12 months, with more than half (52.6%) saying rises are certain and a further 43.5% expecting them to be likely.

The survey also found taxation has overtaken inflation and market volatility as investors’ biggest financial concern.

Almost 45% (44.9%) of respondents said higher taxes posed the greatest threat to their wealth, compared with just 3.5% who cited inflation and the same proportion who identified market volatility as their biggest worry.

The findings come after Burnham secured the backing of 349 Labour MPs, making it mathematically impossible for a rival candidate to enter the leadership contest ahead of his expected move into Downing Street on 20 July.

Susannah Streeter, chief investment strategist at Wealth Club, said: “As Andy Burnham prepares to move into Number 10 Downing Street, investors are already bracing for higher taxes.

“Mr Burnham will inherit one of the toughest fiscal challenges of any incoming Prime Minister in decades. With pressure to repair the public finances, fund public services and stimulate economic growth, investors believe that meeting those competing priorities is likely to require higher taxes.”

Susannah said the survey suggested investors now viewed taxation as a greater risk than many of the economic issues that have dominated recent years.

“Perhaps the most revealing finding is what investors now see as the biggest risk to their wealth,” she said.

“Nearly 45% say higher taxes are their greatest concern, making them by far the biggest perceived threat. By comparison, just 3.5% cite inflation and the same proportion identify market volatility.”

Streeter said investors would be looking for reassurance that any tax changes encourage long-term growth and provide certainty for businesses.

She concluded: “With expectations of further tax rises running so high, now is the time to review financial plans, make full use of available tax allowances and ensure investments are structured as tax-efficiently as possible before any future changes are announced.”