Liz Truss stressed her loyalty to Boris Johnson as she faced questions about why she did not join colleagues in resigning to force the Prime Minister from office.

The Foreign Secretary, the favoured candidate of Mr Johnson’s remaining allies in the party, used her campaign launch to position herself as the most experienced contender to lead the Conservatives into the next general election.

Under pressure after placing third in the first ballot of Tory MPs, Ms Truss sought to stress her credentials on Brexit and defence, as well as offering an implicit swipe at rival Rishi Sunak as she warned now is not the time for “business-as-usual economic management”.

She declined the same opportunity to explicitly criticise Penny Mordaunt, who outpolled her in the ballot on Wednesday evening and is seen as posing a particular threat to the Foreign Secretary.

Instead, she made much of her role in ripping up the Northern Ireland Protocol and standing up to Russia since the invasion of Ukraine.

The campaign to replace the Prime Minister has so far been marked by insurgent campaigns from outsiders and relatively junior contenders, notably Ms Mordaunt and Kemi Badenoch.

Ms Truss, taking questions from reporters, rejected the suggestion that her apparent closeness to Mr Johnson could prove a fatal weakness for her campaign.

To cheers from her campaign team, she said: “I am a loyal person. I am loyal to Boris Johnson. I supported our Prime Minister’s aspirations and I want to deliver the promise of the 2019 manifesto.

“What we need to do now is deliver, deliver, deliver, and I am the person in this race with the record of delivery.”

Her campaign launch came after former Brexit minister Lord Frost launched a scathing attack on Ms Mordaunt, saying as his deputy she had lacked a grasp of the detail and was unwilling to deliver tough messages to Brussels.

Asked about those accusations, Ms Truss said she would not be making any “disparaging comments” about her rival.

“The Conservative race shows what a broad range of talents we have in the Conservative Party. And we didn’t get there through identity politics,” she said.

“We didn’t get there through quotas. We got there because we are a meritocratic party that believes in the future of Britain.”

She told the launch that she wants to see defence spending rise to 3% of GDP by the end of the decade, repeating her pledge to reverse the rise in national insurance and stressing she agrees “completely” with the Government’s controversial Rwanda asylum policy.

The Foreign Secretary has also made the eye-catching suggestion that Government should treat the debt from the Covid-19 pandemic as something to pay off over a longer period of time, offering a way to cover the cost of tax reductions.

In her speech, she said she would be ready to be “prime minister from day one”.

She added: “I will campaign as a Conservative and I will govern as a Conservative.”

She opened her speech with a clear message: “We are at a critical moment for our country.

“Now is the time to be bold, we cannot have business-as-usual economic management, which has led to low growth for decades.”

To shouts of “hear, hear”, she said it is time to deliver on Brexit and “win the fight for freedom” at home and abroad.

In an air conditioned, white-walled room, her backers and campaign team mingled over coffee and croissants.

Former Tory leader Iain Duncan Smith, Culture Secretary Nadine Dorries and Work and Pensions Secretary Therese Coffey were among the high-profile names in the audience, while Business and Energy Secretary Kwasi Kwarteng introduced Ms Truss.

The Foreign Secretary said: “There isn’t a great groundswell of support in the country for (Labour leader) Sir Keir Starmer or (Liberal Democrat leader) Sir Ed Davey.

“What the British people are crying out for is a modern and united Conservative Party, ready with the courage of its convictions to deliver on the promises that we have made.”