Sir Keir Starmer’s former chief of staff has conceded that Labour failed to properly prepare for power in the run-up to its landslide general election win.

In his first media interview, Morgan McSweeney told the BBC’s Nick Robinson he did not yet have all the answers for the prime minister’s dramatic downfall just two years after he led the party back into office.

But he admitted Labour had not given enough thought to how the world had changed since the party last took power in the 1990s.

He added the party should have been “way more optimistic” in its first few months, and had been unable to deliver results quickly enough to satisfy voters.

He told the BBC’s Political Thinking with Nick Robinson podcast: “We didn’t prepare enough for what kind of world we were going to. We are now in a very different era than when Labour was last in government.

“I think we didn’t have enough conversations at the top of the party about what that meant, how to prepare for it, what that meant for the state.

“You have to deliver quite quickly for people, for them to see the change quickly. And I think we didn’t come in with enough of a theory about how we would do that.”

McSweeney ran Labour’s successful 2024 election campaign and followed Sir Keir into office as his head of political strategy.

He has kept a low public profile despite his instrumental role behind the scenes, but was thrust into the limelight earlier this year when he resigned over his role in Peter Mandelson’s appointment as the UK’s ambassador to the US.

McSweeney said he was “still processing” Sir Keir’s political demise, but identified a lack of preparation as a key factor in the government’s early troubles, adding that Labour’s time in opposition “went quickly”.

He said that there had been a widespread expectation that Labour would require at least two elections to return to power after its crushing defeat in 2019, and “quite a lot of people” thought it needed a plan for defeat rather than victory in 2024.

He recalled that during planning meetings early that year, he “did start to realise that we hadn’t done enough to prepare for government”.

McSweeney took on the chief of staff role three months after Labour’s return to office, replacing top civil servant Sue Gray, who was appointed the year before the election and tasked with leading preparations for government.

Asked about Gray’s role in the run-up to taking power, he replied it was “not about one individual”, adding: “When I say we weren’t prepared, I really do mean the Labour Party more generally”.

He added: “I take my own responsibilities for that, rather than blaming one person.”

Sir Keir himself had made arguments about how Britain had changed in the years since Labour’s last term in office, he went on, but “I don’t think that we really discussed what that meant for how we prepare for government”.

Reflecting on Labour’s early months in office, when it complained of the state of the public finances it inherited from the Conservatives, he said the party should have been “way more optimistic when we started”.

He admitted an early decision to remove winter fuel payments from millions of pensioners, a policy on which it would later U-turn, had been a mistake and had “defined the government in a way that did us a lot of damage”.

He said it was not a mistake to means-test winter fuel payments so that better-off pensioners did not receive them, but added the threshold for claiming them had been set at “too low a level”.

He also admitted there was “no question” the party had been damaged by an early row over freebies given to ministers by donors. Sir Keir himself had accepted thousands of pounds’ worth of clothing and spectacles in opposition.

Whilst politicians require a “wardrobe budget” given the need to regularly appear on TV during campaigns, he added, it would have been better for this to have been paid for out of the party’s campaign budget.

Elsewhere in his interview, McSweeney said he found Donald Trump “much funnier than I expected him to be”.

He said that during his first phone call with Sir Keir, officials in the room had been “barely able to contain themselves” with a joke the US president made about foxes eating birds killed by “windmills” – his preferred term for wind turbines.

“He went on to say that as the foxes ate so many birds and became lazy, they became fat, and as they became so fat people no longer knew what kind of a creature they were,” he added.

Asked whether Trump had been trying to be funny, he replied: “Yeah, yeah, yeah. Yeah. Definitely.”

McSweeney also said he thought Andy Burnham, Sir Keir’s expected successor, was the right person to replace the prime minister as Labour leader, adding: “I feel optimistic about it”.

He added that he also backed Burnham’s plan to create a Downing Street unit in Manchester, adding he did not think the idea was just a “gimmick”.

“If at the top of government there are people who don’t just have a desk somewhere outside London but actually live their whole lives outside of London, I think that will be a good thing,” he said.

“A lot of people won’t like it. I think it’s a good idea. I think he should just push it through, the logistics can be sorted out.”

Speaking about the end of Sir Keir’s time in office, he disclosed he had found himself too sad to watch the entirety of the prime minister’s emotional resignation speech in Downing Street last week.

Asked about his own future, he said he wanted to move in a “completely different direction” professionally, adding he had no intention of returning to British politics “in the foreseeable”.

He added: “I mean, I can’t say forever, but certainly for the next few years I’m committing to being out of politics for at least the next few years ahead.”