A new survey of members of the governing Conservative Party shows Foreign Secretary Liz Truss is firmly ahead of rival Rishi Sunak in the race to replace Boris Johnson as the British Prime Minister.

The survey of Tory members who will be electing a new leader to take charge at 10 Downing Street from September 5 released on Wednesday night by the ConservativeHome website found that 58 per cent of those polled back Truss.

Former Chancellor Sunak was found to have the support of 26 per cent, while 12 per cent were undecided. It is the second poll from Wednesday showing the Cabinet minister widening her lead over the British Indian ex-minister, with an earlier YouGov poll showing Truss was ahead among all age groups, across different parts of the country and among men and women.

"YouGov gives her a 34 point lead which extends, on a forced choice, to a 38 point lead (69 per cent to 31 per cent). Divide our 16 points of don’t knows and others evenly between the candidates… Truss has a 32 point lead (66 per cent to 34 per cent),” the ConservativeHome survey claims.

“All in all, Sunak needs a massive game-changer to turn this contest round if our new findings and YouGov’s are right. And it’s very difficult to see where that could come from,” it notes.

The latest findings come as Sunak faced a fresh blow with another former candidate and senior Tory, Sajid Javid, endorsing Truss in the race for her “bold agenda”.

The Pakistani-origin former Health Secretary, and also Sunak’s former boss in the UK Treasury when he was the Chancellor, writes in ‘The Times’ that a refusal to cut taxes meant the UK risks "sleepwalking into a high-tax, low-growth economy" – a reference to Sunak’s tax strategy.

"Some claim that tax cuts can only come once we have growth. I believe the exact opposite – tax cuts are a prerequisite for growth,” writes Javid.

"Tax cuts now are essential. There are no risk-free options in government. However, in my view, not cutting taxes carries an even greater risk," he said.

Last month, Javid had launched a leadership bid of his own for the top job on a platform of swift tax cuts but pulled out after he failed to win enough nominations from Tory MPs to go through to the first round of voting.

It was his resignation from Johnson’s Cabinet that came first, followed soon after by Sunak and then several other ministers that forced the outgoing prime minister’s hand to announce his resignation and trigger a Tory leadership election.

Truss now has the backing of most former candidates, including Chancellor Nadhim Zahawi, Trade Minister Penny Mordaunt and Tory backbencher Tom Tugendhat.

An estimated 180,000 party members started receiving their ballot papers this week and have the option of a postal or online ballot, which must be registered by the evening of September 2. The votes will then be counted by the Conservative Campaign Headquarters (CCHQ) and results declared on September 5.