Speaking to the Calling Peston podcast, he attributed Mr Johnson's popularity to him being "really lucky" in having the NHS rollout a successful coronavirus vaccination programme and the Treasury's furlough scheme.
“The point I’m making is that the two things that people are most concerned about in the last 18 months, as I say their health and basically their job, have been covered by either vaccine or furlough. We’re coming to the end of those.”
He suggested the public will be ready to punish the prime minister for his "broken" promises by the time of the next election and people will be looking for a leader with integrity.
Sir Keir Starmer believes the road will run out on Johnson's luck:
“I don’t buy into this argument that character just doesn’t matter," he said.
"I do think that in the end integrity honesty and accountability do matter and I think there will come a point at which the road will run out for Johnson on this."
In an exclusive interview to mark the Calling Peston podcast's 100th episode, Sir Keir said he believes Mr Johnson will seek to hold a general election in May 2023, "before all of his broken promises catch up with him", but he will be ready as soon as next year.
He said: "I think Johnson will want to go to the country before all of his broken promises catch up with him and May 2023 seems to me, looking at all the economic outlooks etc, to be a time when he might go to the country - if he goes earlier than that we’ll be ready.
“I’ve put the party on a footing to be ready by May 2023, fully ready.
“But we’ll be ready if he goes before that. If it goes the full distance well it goes the full distance but I’m anticipating that we’ll have to fight in May 2023."
Sir Keir did not rule out forming a pre-election coalitions with left leaning parties, when asked by Robert Peston if there was any potential for joining forces.
Starmer: 'We'll be ready for an election as soon as one is called':
“We've got to do a lot more work, you know, obviously going into the next general election," he said, "there will be a question of what we do."
“There's a majority broadly against the Tories in the country and obviously we'll have to see how we go into the next general election, but, you know, the rules are what they are, but we're rebuilding our party, putting out our arguments and going hell for leather for that election probably on May 23, I suspect.”
When asked, Sir Keir joked that he could use Gary Neville and other footballers in his shadow cabinet, with players united in their desire to tackle the government.
"Oh yeah, we’ll have Gary - I think there’s quite a few in the England team who would equally want to be in the shadow cabinet, so we’ll have a sporting shadow cabinet with all of these stars in.
'We'll have Gary!'
“Gary’s been very outspoken and he’s been very critical, rightly, of the prime minister," he said.
Sir Keir said he would unveil his vision for the UK under his leadership in September this year.
"We’re going to set it out at party conference, but in very broad terms I want a country that we can all be proud of because of our success, whether that’s economically, with the future of our economy, whether its internationally, whether for that matter whether its culture, art or sport."
He added: "A country we’re proud of because we look after each other and we harness the spirit of the pandemic and embrace community and bringing people together and that includes a big argument about the future of public services and a country we can be proud of because we treat everyone with respect and dignity."
Listen to the full interview with Starmer in the Calling Peston podcast's 100th episode: