Jeremy Corbyn's leadership, unclear messaging and a toxic culture have been blamed for Labour's most recent General Election failings.
Labour Together, a group including MPs Ed Miliband and Lucy Powell, party members, union figures and people from the media, examined the defeat it said was "a long time coming".
The group's report sets out “political, organisational and digital” failures which resulted in Labour's worst General Election performance since 1935, while trying to chart the party's path back to power.
The party warned it has a "mountain to climb” to win the next election and that Sir Keir Starmer's leadership won't be enough to catapult Labour into Number 10.
The report said under Mr Corbyn, Labour was "unprepared" for a general election, had "no clear message" and its manifesto pledges were seen as impossible to deliver by the public.
It added: “There is a broad consensus across the Party – mirrored in the results from the survey of Labour members – that a combination of concerns about the leadership, Labour’s position on Brexit and Labour’s manifesto not being seen as deliverable damaged Labour’s chances in the election.
“The Commission concludes that the weaknesses going into this election were interlinked, and indivisible. They catalysed long term trends between Labour and its voter coalition.”
The report also warned that making Sir Keir Starmer leader would not be enough to help them win the next General Election unless underlying issues from the previous leadership, including anti-Semitism, were not addressed.
“Concerns about Labour’s leadership were a significant factor in the election loss in 2019,” the report said.
“‘Stop Jeremy Corbyn’ was a major driver of the Conservatives’ success across all their key groups including previous non-voters, and among all the swing voters Labour lost to the Tories.
“The very low poll ratings on leadership going into the 2019 election cannot easily be disentangled from the handling of issues like Brexit, party disunity and anti-Semitism.”
It urged different factions within the party to unite in order to stand a chance against the Conservatives. Lack of planning was also cited, saying the party lacked a "clear strategy of which voters we needed to persuade or how".
Labour was also outperformed on both traditional and social media, with research for the review showing the Conservatives “invested heavily in digital” and had far more success with organic shares of their messaging, while making “much better use” of Facebook groups and other online forums.
By comparison, Labour activists were plagued by “crashing digital tools that created more work not less for candidates and campaign teams”, and by a lack of “best practice messaging”.
The review also showed Labour lost many voters due to Brexit, with more deserting the party before the referendum than in last year’s election.
Among recommendations, the group called for a “coherent strategy to build a winning coalition” at the next election and “a renewed commitment to transformational economic change” in Britain, rooted in a credible understanding in the “struggles of people’s lives”.
The report said Labour should build “a culture of inclusion and diversity, generosity and teamwork, not factionalism and patronage in the party”.
It also called for a “root and branch reform of our party organisation and structures, bringing it into the 2020s, so it connects better to the communities and voters that we seek to serve, including a wholesale transformation of our digital and online campaigning”.