Rishi Sunak, one of the two contenders to replace British Prime Minister Boris Johnson, has defended his decision to quit the government and help end Mr Johnson’s scandal-tarnished leadership.

Allies of Mr Johnson accuse Mr Sunak of treachery for stepping down as Chancellor of the Exchequer – or treasurer – last month as the government floundered amid ethics scandals, with a resignation letter, saying ‘‘we cannot continue like this’’.

More than 50 government ministers went on to quit, leaving Mr Johnson no choice but to resign as Conservative Party leader and prime minister of the country.

Polls of Conservative members put Foreign Secretary Liz Truss ahead of Mr Sunak in the contest to succeed Mr Johnson as party leader.

Postal ballots started arriving on Monday at the homes of about 180,000 party members who will decide the winner.

The new party leader will be announced on September 5 and will automatically become prime minister.

Mr Sunak said his critics were ‘‘looking at the last few months of the government with slightly rose-tinted glasses’’.

‘‘The government found itself on the wrong side of a very serious ethical issue,’’ he said.

Mr Johnson led the Conservatives to a commanding parliamentary majority in 2019, but was subsequently plagued by scandals, and was fined by police over parties at his official office and residence that broke COVID-19 lockdown rules.

He clung to power for months but finally quit on July 7 after one scandal too many – appointing a politician accused of sexual misconduct – drove his ministers to resign en masse.

Mr Sunak is running as an experienced minister who can guide the country through the economic turbulence caused by the COVID-19 pandemic and the war in Ukraine.

He was the favourite candidate among Tory MPs who whittled down the field of candidates from an initial 11 contenders.

But Ms Truss appears to have more momentum with the rank-and-file party members and has won backing from several high-profile MPs and cabinet ministers with her pledges to slash taxes and government bureaucracy.

Mr Sunak’s replacement as finance minister, Nadhim Zahawi, said he was backing Ms Truss because she offered a ‘‘booster’’ economic approach compared to ‘‘doomster’ Mr Sunak.

Mr Sunak accuses Ms Truss of peddling unrealistic promises, especially on the economy.

Ms Truss says she will cut taxes immediately to ease the cost-of-living crisis, funding it through borrowing.

Mr Sunak argues it is vital to get inflation under control first.

But on Monday he pledged a big income tax cut in the next few years, in what critics said was a U-turn forced by pressure from Ms Truss’s campaign.

Mr Sunak said he would cut the basic rate of income tax from 20 per cent to 16 per cent by 2029.

‘‘I want to make sure that we can pay for it. I want to make sure that we can do it alongside growing the economy,’’ he said.

-AP