Sir Keir Starmer has admitted that Labour "got it wrong" on migration - and "shied away" from the concerns of the British public.
The Prime Minister first wrote in The Telegraph and then echoed his message in a major conference of some of the West's left-wing leaders.
He acknowledged that migration was "where some on the left went wrong".
Sir Keir also claimed that Labour was being forced to counter the "rise of the populist right" in a speech at the Global Progress Action Summit today.
Previewing what he later went on to say, he wrote in his piece: "There is no doubt that for years left-wing parties, including my own, did shy away from people's concerns around illegal immigration.
"It has been too easy for people to enter the country, work in the shadow economy and remain illegally."
Sir Keir also lashed out at the poll-topping Reform UK - whose "simple slogans will not do anything to resolve" the migrant crisis.
"There is no silver bullet, but we must enforce every possible measure to deter illegal migrants from entering British waters," he added.
Some 1,157 migrants crossed the Channel over the last week.
LATEST DEVELOPMENTS:
Prime Minister Sir Keir Starmer delivers a speech to the Global Progress Action Summit, at Methodist Central Hall in Westminster
|PA
But echoing Reform's attacks on the "Boriswave" migration surge, the PM added: "The huge increase in immigration that happened under the Conservatives was based on a hyper-liberal free market viewpoint.
"Labour is clear that there must be no return to that."
He finished: "Britain faces a choice between decency and division, between renewal and decline.
"This is the defining political choice of our times, and we must be unafraid of challenging a politics of grievance, preying on the problems of working people."
During his speech, Prime Minister Starmer claimed Britain faced a 'battle for the soul' of the country as he pledged a fightback against the threat of Reform UK
|PA
During his speech, Prime Minister Starmer claimed Britain faced a "battle for the soul" of the country as he pledged a fightback against the threat of Reform UK.
In a rallying cry to progressives, the Prime Minister said centre-left leaders must stop the "politics of predatory grievance" from "preying on the problems of working people".
The intervention comes amid concerns within Labour that the Government has failed to be passionate enough in its attacks against Nigel Farage’s insurgent right-wing party.
Speaking alongside counterparts from Canada, Iceland and Australia at the Global Progress Action summit on Friday, Sir Keir criticised the “poisonous belief" that a "violent struggle" for the nation is coming.
Keir Starmer said the era of two-party politics dominated by Labour and the Tories is changing
|PA
He again condemned scenes earlier this month at the Unite the Kingdom rally organised by far-right agitator Tommy Robinson, real name Stephen Yaxley Lennon, which Sir Keir said sent "shivers” through many communities.
He said the era of two-party politics dominated by Labour and the Tories is changing, with the ruling party now up against “a very different" challenge from Reform.
"The UK (is) now at a crossroads, we have got a right-wing proposition that we’ve not had in this country before," he said.
Setting the tone for Labour’s annual conference this weekend, he said the next election would be an “open fight" between the ruling party and Reform – an argument he will expand on in his leaders’ speech in Liverpool.