Rory Stewart has warned Tory leadership rivals against offering “cheap electoral bribes” to win support, saying party members who will choose the next PM are “smarter than this”.
Rather than being “straight” with people, he said opponents had pledged “eye-watering” tax cuts worth £84bn.
Boris Johnson kicked off his campaign by pledging an income tax cut for those who earn more than £50,000 a year.
He has won the endorsement of former party leader Iain Duncan Smith.
Writing in the Daily Telegraph, Mr Duncan Smith said the ex-London Mayor was best placed to get the UK out of the EU by the 31 October deadline and generate a “mood of optimism” around the party.
Ten Conservative candidates will contest Thursday’s first round of voting after nominations closed in the contest to succeed Theresa May as Tory leader and prime minister.
Sam Gyimah was the only candidate to pull out as all the others achieved the eight supporters that were necessary to get themselves on to the first ballot paper.
He told BBC Radio 4’s Today programme he had entered the leadership race late while others had been building support for more than a year.
He said all 10 candidates had contacted him overnight seeking his support but he would not decide until he knew their Plan B for achieving Brexit.
- Final 10 named for Tory leadership race
- The UK’s next prime minister: What you need to know
- Where do the candidates stand on Brexit?
- Do Tory leadership tax plans add up?
Over the next two weeks, Tory MPs will take part in a series of secret ballots to whittle the candidates down to the final two. The party’s 160,000 or so members will then pick a winner in a postal ballot, with the result being announced in the penultimate week of July.
The first full day of the official campaign will see other candidates make their opening pitches:
- Andrea Leadsom is expected to call for a “managed exit” from the EU and action to tackle the climate emergency
- Former chief whip Mark Harper will also launch his campaign, promising a “fresh approach” to Brexit and more spending on education
- Mr Stewart, who recently joined the cabinet as international development secretary, will warn against “reckless” tax and spending pledges at an official campaign launch later.
On Monday, Mr Johnson said he would use money currently set aside for a no-deal Brexit to raise the 40% tax rate threshold to £80,000.
Mr Stewart will say his party’s reputation for economic prudence is at risk and, while a negotiated Brexit deal will create some “headroom” for extra spending, any money available should be spent on technical education and digital infrastructure.
“We have to think about the next 15 years, not the next 15 days,” he is expected to say.
“We cannot criticise Jeremy Corbyn for reckless spending pledges if we start doing the same ourselves. Cheap electoral bribes could cost us dear. Our members are smarter than that.”
Speaking on BBC Two’s Newsnight programme, Mr Harper said he believed Mr Johnson’s tax plans were the “wrong choice” for the country and any tax cuts should be targeted at low-earners and those on universal credit.
On Brexit, he said those who had sat in cabinet over the past three years, including Mr Johnson, Michael Gove, Jeremy Hunt and Sajid Javid, shared responsibility for the government’s mistakes and an outsider was best placed to achieve a breakthrough in the talks.
The Johnson factor
The front-runner’s team is actively trying to keep him out of it for now.
There is a very concerted effort by Boris Johnson’s team not to stuff up his initial advantage – in the words of one, to “stop him doing mad things”.
But it’s not so strange, in a way, because that is how Mr Johnson’s supporters argue he would run the government.
The idea is that he could provide the pizzazz and profile to energise his party, and the government.
But the day-to-day grind of running the government would be down to others. The model I’ve heard it described countless times, he would be the chairman, not the chief executive.
Any politician who tells you they wouldn’t like to have even a sprinkle of the former foreign secretary’s stardust is likely not to be telling you the truth. For good or ill, whether they love or hate him, he is not a politician that is easy to ignore.
And that’s why MPs who think some of his ideas are bonkers are now signed up to be part of his team. They believe he can win, and they want to be part of it, if even to restrain his excess.
However, Mr Johnson – who has yet to a do a major campaign appearance or TV interview – continues to win the backing of MPs from different wings of the party, such as prisons minister Robert Buckland, a leading proponent of a soft Brexit.
Mrs May officially stepped down as the leader of the Conservative Party last week, but will remain as prime minister until her successor is chosen.
Who will replace Theresa May?
The winner of the contest to lead the Conservative Party will become the next prime minister.
All 313 Conservative MPs will vote for their preferred candidate in a series of ballots held on 13, 18, 19 and 20 June to whittle down the contenders one by one until only two are left.
Candidates will need to win the votes of at least 16 other MPs in the first ballot and 32 colleagues in the second to proceed.
The final two will be put to the 160,000 or so members of the wider Conservative Party in a vote from 22 June, with the winner expected to be announced about four weeks later.
On Tuesday 18 June BBC One will be hosting a live election debate between the Conservative MPs who are still in the race.
If you would like to ask the candidates a question live on air, use the form below. It should be open to all of them, not a specific politician.
If you are reading this page on the BBC News app, you will need to visit the mobile version of the BBC website to submit your question on this topic.
Tory leadership contest: Rory Stewart warns over ‘electoral bribes’}