Boris Johnson is building support in the Tory leadership race with the backing of defence secretary, Ben Wallace, who ruled himself out of the contest. Wallace, who is popular with Conservative members, said he “would lean towards” supporting Johnson. “I think he will still have some questions to answer about that [Partygate] investigation,” Wallace said, but added: “He got a mandate and I think that’s an important thing for all of us to bear in mind.” The former prime minister, who is returning from his Caribbean holiday, is testing the waters with supporters on Friday in an attempt to reach the 100 MP threshold required. There are about 40 publicly declared Johnson supporters or sympathisers and his allies believe he is more than halfway towards the target. Rishi Sunak, the other frontrunner, is believed to have even more Tory MP supporters, with 57 declared, but the Conservative membership will ultimately decide between the final candidates. Jacob Rees-Mogg, the business secretary, Simon Clarke, the levelling-up secretary and Nadine Dorries, the former culture secretary, are some of Johnson’s most prominent declared backers, while Sunak has the support of Mel Stride, the Treasury committee chair, and Steve Barclay, Johnson’s former chief of staff. Penny Mordaunt, another former leadership contender, is also considering running again. Despite the MPs supporting Johnson, he still faces a privileges committee investigation into his conduct for allegedly lying to the House of Commons over the Partygate scandal, and some Conservative MPs would be likely to refuse to serve under him. Jesse Norman, a senior Tory MP and Foreign Office minister, warned that returning to Johnson would be a disaster. “There are several very good potential candidates for Conservative leader. But choosing Boris now would be — and I say this advisedly — an absolutely catastrophic decision,” he said. George Eustice, the former environment secretary who was loyal to Johnson until the end, also questioned the ex-prime minister’s ability to make a return and threw his weight behind Rishi Sunak. “The big question is, is it possible just two months later, for him to return and genuinely lead and rekindle the trust of those two thirds of Conservative MPs who were willing to vote against him as recently as two months ago?” he told Times Radio. “I mean, half of his own ministers quit his government on the final day, because they all said that they’d lost confidence in him, and I really can’t see how it’s possible for him to return.” The idea that Johnson could return to No 10 was beginning to sink in with Conservative MPs who ousted him. One senior backbench Tory said: “It looks to me like he’ll get the 100 nominations. And with the membership, I think it would be Boris. I really do. What has been instructive is the amount of emails I’ve had from my locals, not all of them party members admittedly, saying bring back Boris. “There was quite a lot of anger over how he was deposed. So if it comes down to the members, I think he’s back in No 10.”