Labour does not "deserve" to win a general election if it refuses to admit it should have done better in local results, according to Alastair Campbell.
The former Downing Street spin doctor told Sky News that he "didn't think anybody could take comfort" from the party's performance this week.
Labour made some gains in the local elections on Thursday, taking Plymouth from the Tories, but failed to win other key target seats.
"I don't think anybody can take comfort from these results, and I don't think anybody should," he said on Saturday.
"If you won't even admit that these results are not as good as they should be, that they are not a winning position for the general election, then I'm sorry.
"If you won't even make the analysis and try and come up with the answers, then frankly I don't think you deserve to win."
He praised Jeremy Corbyn for "bringing more young people into the party" and acknowledged he fought "a much better campaign than we thought he would".
But Mr Campbell blasted the Labour leader's team for a "terrible complacency about their attitude to politics".
He said: "They basically think 'if only everyone would just shut up, get behind Jeremy and say that he's marvellous, then Labour's going to waltz into power'.
"Politics does not work like that."
Earlier, in a conference speech to the centre-left pressure group Progress, he lamented that Labour was "a long way from where we need to be".
He added that "huge swathes of the country" cannot accept "this Labour party" in power.
Mr Campbell, who served as communications director under Tony Blair, also referenced a soundbite of the era to say Labour should be "tough on Brexit, tough on the causes of Brexit".
He added that if Labour's Brexit tests are to mean anything, the party cannot support Prime Minister Theresa May's strategy.
"If they do, then though the Tories will be seen as Brexit's architect, Labour will be its bricklayer and history harsh when the house comes tumbling down," he said.
Labour MP Stephen Doughty told Sky News that "both the parties would have liked to have done better" in Thursday's poll.
He welcomed Labour's clarity on remaining in a customs union after Brexit, adding: "I want to see us go further and back a people's vote."
Mr Corbyn has hailed the local election results as "building on the historic gains" Labour made in last year's general election.
He predicted that with activists' support "we can and will form a government".