The Tories should have campaigned for change rather than "continuity" in the General Election, Theresa May's former chief of staff has said.
Nick Timothy, who was the Prime Minister's joint chief of staff alongside Fiona Hill, claimed the party's electoral strategy was "wrong" on reflection.
He told the Daily Telegraph Mrs May's leadership had been based on the idea that the Brexit vote was a "vote for serious change", and that under her leadership, the Tories were conveying a message that was "on the side of change".
"But then having done that the electoral strategy was fundamentally different. It was a reassurance and continuity campaign rather than a change campaign and on reflection I think that was wrong."
Mr Timothy, who resigned in the wake of the disastrous showing by the Conservatives in the election, said the party would have been "much better off with a message showing we understood the need for change and we were the people capable of delivering it".
However, he denied that the "strong and stable" slogan was a "necessarily a problem".
He told the paper the Tories clearly underestimated the Labour leader Jeremy Corbyn, and conceded that it "probably is true that there should have been more on the economy during the campaign".
Mr Timothy warned if the Conservatives become "much more orthodox" in the future that it could lead to a "dangerous left-wing alternative" being elected.
He said: "Overall the lesson of the election for the party and for the Government cannot be 'Oh well, we tried that and we didn't win the election we were hoping for so let's not try it any more'.
"If the party retreats to a much more orthodox Conservative proposition then I worry that won't be sufficient to tackle the big problems that the country has and in five years' time we do risk the election of a dangerous left-wing alternative."
There was widespread criticism within Conservative ranks over Mr Timothy and Ms Hill's part in the party's election campaign, but Mr Timothy said he was still in touch with the Prime Minister.
"I have spoken to Theresa a few times since the election but I haven't seen her and I'm not advising her on policy.
"They are private conversations, people catching up."
Mr Timothy also dismissed speculation that Mrs May wanted to oust Chancellor Philip Hammond after the election, insisting they "get along fine".
"She did not intend to get rid of him," he said.
"Theresa refused to even talk about post-election reshuffles because she thought it was inappropriate, it took things for granted, she wanted to concentrate on the campaign, so that was just never on the cards and I don't know what the origin of that story was.
"They go for dinner or breakfast with one another probably every fortnight, they get along fine but the two of them are businesslike politicians, that's how they work.
"It's the cliche about her that she has never sought to be in a gang or have her own gang, but the way people perceive the relationship is in part down to the way the two of them tend to conduct themselves. There is no rift."