Britain's transport secretary on Saturday became the fifth Conservative MP to launch a bid to succeed Prime Minister Boris Johnson, a long-shot addition to the growing and already acrimonious leadership race.

Grant Shapps, an experienced lawmaker who first served in the cabinet under former premier David Cameron back in 2010 but not among the current frontrunners in the polls to replace Johnson, vowed to provide "strategic" and "sober" government.

His announcement came hours after Defence Secretary Ben Wallace, who has impressed in the role amid the war in Ukraine and has been one of Tory members' favourites in several recent surveys, said he will not run after discussing standing with colleagues and family.

"It has not been an easy choice to make, but my focus is on my current job and keeping this great country safe," he added on Twitter.

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.

- Tax debate -
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 resigned late Tuesday, triggering dozens of more junior colleagues to follow suit and forcing his ex-boss to then quit as Tory leader 36 hours later.

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

A summer of rancorous campaigning now looms. 

Party members will eventually choose 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.

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.

- 'Plotting' -
Alongside Sunak, attorney general and arch-Brexiteer Suella Braverman, the relatively unknown former equalities minister Kemi Badenoch and backbench Tory MP Tom Tugendhat have announced their candidacies.

Foreign Secretary Liz Truss and new finance minister Nadhim Zahawi -- who replaced Sunak in the treasury -- are expected to join the crowded field.

Former foreign secretary Jeremy Hunt, who was runner-up to Boris Johnson in 2019, is also "virtually certain" to run again, allies told UK media.

Former finance and health minister Sajid Javid, who also quit Johnson's government Tuesday, may also stand but had been asked by Sunak's allies to step aside to give him a clear run at the leadership, The Times reported.

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 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".

- Interim government -
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 put to members be decided within weeks, before parliament's summer recess starting after July 21.