A damning internal report into Labour’s on-the-ground election campaign has revealed activists did not have enough campaign material, were poorly prepared to talk about policy on the doorstep and did not target the right marginal seats.

The leaked analysis by a Labour staffer, produced for senior party figures, found that officials were not in the right parts of the country to run an effective strategy, despite the fact that door-to-door campaigning increases turnout for Labour.

And though Labour is the largest political party in Europe, the report – seen by the Guardian – suggests it needs to expand its membership even further if it is to swing decisive seats in future elections.

“Discussions with activists reveal … major deficiencies in the running of the campaign,” the report reveals. “These include poor quality and late delivery of printed materials, lack of communication and preparation over key policy lines, lack of clarity on how campaigns could secure the data of newly registered voters, the unreliability of digital tools, and inadequate training of activists.

“Therefore there is clearly major room for improvement in the effectiveness of on-the-ground campaigning.”

Significant regional differences were also revealed in the rate of conversations Labour activists had with voters. While London marginals recorded that there had been 23,000 “contacts” (28% of eligible voters), in Scottish marginals there was a contact rate of just 6.4%, or 4,745 people.

Of the 135 key marginals the staffer analysed, a contact rate of more than 10% of voters was not reached in 56 of them. The report said the constituencies were “either not targeted or targeted weakly”.

Only 25 constituency Labour parties achieved contact rates of more than 20%.

A Labour source said: “What this report shows is that canvassing does work and it does help but the party was not really well organised and there were basic deficiencies that activists weren’t getting data and there should have been different approaches to conversations at different points in the campaign. But there is statistical proof that canvassing works.”

The report found that for every 10% change in campaign contact rate there would be a 2% increase in Labour’s vote share.

The report is one of a number of critical election postmortems for the Labour party, which returned its worst result since 1935.

So far an analysis presented to the party’s National Executive Committee found that the poor election performance was down to the party leadership’s decision to back a second Brexit referendum and that too many policies were released on a daily basis throughout the election period.

It did not blame the leader, Jeremy Corbyn, who they suggested had been the victim of unrelenting attacks on his character.

Organisation Labour Together – which is being backed by the party’s former leader Ed Miliband and the Manchester Central MP Lucy Powell – has so far had feedback from 8,000 people. Early findings showed that voters did not like the party’s policies, and that the campaign was inefficient and beset with technical difficulties.

However, this fresh report seen by the Guardian is the first statistical analysis of how conversations with voters correlated to vote share for the party in individual seats.

Citing the link between the contact rate and the increase in the Labour vote share, the report said: “The party’s potential to mobilise large numbers of volunteers is a major electoral asset, with significant potential to swing decisive seats if [it] targets accurately. To maximise on this however, the party needs to retrain and expand its membership, particularly in areas currently lacking.”

As party members dissect the election result, activists who went door-to-door are keen for the role of canvassing to be recognised by the party centrally as a powerful way to secure votes and that it should be improved in time for the next election.

They suggest more work needs to be done on the types of conversations to have with voters at different stages of the campaign. There were complaints that in some seats there was only a “voter ID” stage, which is when activists go house to house to gather if anyone might be a Labour supporter. Instead there needed to be more focus on the “get the vote out” phase and countering opponents’ attack lines.

A Labour party spokesman said: “The National Executive Committee is collecting evidence from all teams and sections of the party for a detailed review of our election campaign and the result.

“We are committed to learning the lessons when that review is complete.”