A trio of Conservative heavyweights, including former health ministers Sajid Javid and Jeremy Hunt, late Saturday announced their bids to succeed Prime Minister Boris Johnson, meaning eight contenders have entered the already acrimonious leadership race.

Javid, also a former finance minister, and Hunt, who finished runner-up to Johnson in the last contest in 2019, were joined on the growing candidate list by current Chancellor of the Exchequer Nadhim Zahawi, who was only appointed to that post on Tuesday.

None of the trio are frontrunners in recent polls of Conservative party members who will ultimately choose their new leader and Johnson's replacement, but are among the most high-profile to have launched campaigns so far.

Hours earlier, Transport Secretary Grant Shapps said he would run, adding yet another candidate to the typically unpredictable political contest. Shapps is an experienced lawmaker who first served in the cabinet in 2010, but not a frontrunner in the polls either.

But Defence Secretary Ben Wallace, who has impressed in the role and been one of Tory members' favourites in several recent surveys, announced he would not stand after a discussion with colleagues and family.

10 Downing Street is the official residence of Britain's prime minister in London
10 Downing Street is the official residence of Britain's prime minister in London AFP / CARLOS JASSO

The likely months-long campaign, potentially pitting more than a dozen Conservative MPs and multiple factions of the ruling party against each another, is set to be formalised Monday when a committee of backbenchers will meet to agree the timetable and rules.

The early frontrunner is former finance minister Rishi Sunak, who helped kickstart the cabinet revolt that led to Johnson's forced resignation on Thursday.

Sunak and Javid both resigned late Tuesday, triggering dozens of more junior colleagues to follow suit and forcing their ex-boss to then quit as Tory leader 36 hours later.

Rishi Sunak resigned on Tuesday as finance minister under Prime Minister Boris Johnson
Rishi Sunak resigned on Tuesday as finance minister under Prime Minister Boris Johnson POOL via AFP / JUSTIN TALLIS

But Johnson, whose three-year premiership has been defined by scandal, the country's departure from the European Union and the Covid pandemic, said he would stay on until his successor is selected.

Party members will eventually select their new leader -- from a two-person shortlist whittled down in multiple rounds of voting by all 358 Tory MPs -- before the Conservatives' annual conference in early October.

Attorney general and arch-Brexiteer Suella Braverman, the relatively unknown former equalities minister Kemi Badenoch, and backbench Tory MP Tom Tugendhat have also announced their candidacies.

Foreign Secretary Liz Truss is among those expected to still join the crowded field.

Taxation is set to be a key feature of the race, alongside candidates' Brexit credentials, as Britain faces the toxic combination of high inflation and rampant cost-of-living increases alongside stagnant growth and relatively high tax rates.

Britain's Foreign Secretary Liz Truss is expected to join the crowded field of contenders
Britain's Foreign Secretary Liz Truss is expected to join the crowded field of contenders AFP / JUSTIN TALLIS

Announcing their bids separately in the Sunday Telegraph, Javid and Hunt both vowed to cut corporation tax from 25 to 15 percent.

Javid said he would also slash or change other taxes, including reversing a recent rise in national insurance that is ringfenced to raise health service funding.

Zahawi also pledged to lower taxes for individuals, families and business, boost defence spending, and continue with education reforms that he started in his previous role heading that government department.

Sunak, narrowly ahead of Truss atop the latest poll of party members, drew immediate support from several senior MPs after declaring he was standing in a slick video on social media late Friday.

He has also been attacked by Johnson loyalists and rival candidates in a sign of the acrimony that could blight the contest.

The Financial Times said Saturday there was "huge anger" within the outgoing prime minister's team at Sunak over his resignation, with a senior official calling him "a treacherous bastard".

In a veiled swipe at Sunak, Shapps said in his leadership announcement that he had "not spent the last few turbulent years plotting or briefing against the prime minister... (or) mobilising a leadership campaign behind his back".

Following the nearly 60 resignations that triggered his decision to quit, Johnson assembled a new team to govern in the interim, announcing a flurry of junior appointments late Friday.

At a first meeting of his hastily convened top ministers, the 58-year-old conceded Thursday that "major fiscal decisions should be left for the next prime minister", Downing Street said.

The Conservatives have declined to say how many eligible members they have, but note it will be more than the 160,000 who voted at the last leadership contest in 2019.

As the list of candidates grows, some senior lawmakers have warned the field must narrow quickly and suggested that the final two-person shortlist to be put to members should be decided within weeks, before parliament's summer recess starting after July 21.