Boris Johnson is facing calls to ensure all evidence on the Downing Street parties is published with the Sue Gray inquiry, as it emerged the pivotal report is likely to amount to a concise summary of findings.

Labour and the Liberal Democrats called on Friday for the report to be published along with its accompanying evidence – including emails and witness accounts – to give full transparency around more than 15 alleged parties under investigation by Gray, a senior civil servant.

The publication is expected to have huge political ramifications as many Conservative MPs have said they will await its findings before deciding whether to back Johnson to stay on as prime minister. If they are unsatisfied, he could face a no-confidence ballot.

Government sources said the report was likely to be ready at some point in the middle to end of next week, and Gray would hand it to No 10. It is understood Johnson will get advance sight of the report but then be expected to make it available to the public and parliament within hours.

The terms of reference of the investigation make it clear that “findings” will be made public. However, the Guardian understands that does not include accompanying evidence such as emails, text messages or transcripts of interviews, or precise details about what happened at any of the alleged gatherings.

Government sources pointed to the outcomes of the investigations into Priti Patel, the home secretary, and Damian Green, the former deputy prime minister, as examples of how such findings tend to be set out. Both ran to less than two pages.

The investigation file, including the evidence on which the findings rely, is unlikely ever to be published, the sources said.

This may mean an email allegedly sent by a senior official warning Johnson’s principal private secretary, Martin Reynolds, not to have drinks in the No 10 garden on 20 May 2020 may never be made public.

The email is crucial, as Johnson insists he was not warned that the “bring your own booze” event might be against the rules, and that he was unaware it was a party when he spent about 25 minutes there speaking to staff, believed to number 30 to 40.

The findings will instead be statements of fact about what happened, while leaving the matter of any disciplinary action to the civil service and the prime minister. Johnson could decide to refer himself and any other ministers to the independent adviser on ministerial interests, Lord Geidt. Redactions from the findings of the names of junior staff and the potential for any disciplinary action against them are also possible.

Angela Rayner, the deputy leader of the Labour party, called for a greater degree of transparency around the parties, which have provoked outrage across the country, from members of the public to Tory MPs.

“Boris Johnson cannot be allowed to cover up or obscure any of the truth when he has insisted on a hugely protracted internal probe to tell him which parties he attended and what happened in his own home. The Sue Gray report must be published in its entirety with all accompanying evidence,” she said.

Rayner said transparency in government has been eroded under the prime minister. “The Conservatives have shown us how little respect they have for the rules, we’ve seen private WhatsApps, missing phones, a freedom of information clearing house, lost minutes of lobbying meetings – their cover-up culture has lost the trust of the British public.”

Ed Davey, the Lib Dem leader, also joined calls for more transparency around the report. He said: “Trust is at an all-time low, so this report must be open to scrutiny from all those who’ve lost loved ones and all those who stuck to the rules. Aside from personnel and employment issues, Boris Johnson owes it to parliament, and above all to the people, to publish this report and the transcripts in full. Anything else will be seen as the usual lies and bending of the rules.”

Despite the decision not to publish accompanying evidence, several current and former civil servants who know Gray said they believed the report would still be an accurate and potentially damning account of the parties that would not shy away from difficult conclusions.

They pointed to the Green investigation, which she led as head of the propriety and ethics team, and resulted in his resignation in 2017. However, other former civil servants highlighted her ability to navigate her way through tricky political problems, which has led to the nickname “Sue Gray Area”.

Gray’s investigation has consistently been described as “independent” despite her being a senior civil servant reporting to the prime minister.

Asked whether the probe was independent, Johnson’s deputy official spokesman said on Friday: “Yes, it is. It is for that inquiry, that team, to establish the facts, we’ve said before … it’s an independent investigation team, I think we’ve set that out from the start.”

The spokesman was asked what about the inquiry made it independent, and he said: “Well, as we’ve set out, it’s being run independently by a civil servant who’s been asked to establish the facts.”

On Friday night new details emerged about the two parties allegedly held in No 10 on 16 April last year for the departure of James Slack, then Johnson’s director of communications, and a Downing Street photographer. Slack has since apologised “unreservedly”.

About 30 people attended both gatherings, with the photographer’s taking place in the basement of No 10 while Slack’s colleagues met in the press area. Both groups later met up in the garden, the Daily Telegraph reported. The newspaper was also shown a photograph of revellers in the basement, although it is unknown if this or any related texts have been seen by Gray.

The basement party reportedly went on for at least seven hours until 1am, according to text messages seen by the Telegraph. Wine was spilt on a government printer as music bellowed from a laptop. A takeaway pizza was ordered into No 10, it is alleged, with slices handed around the garden – while other partygoers took turns on a slide bought for the Johnsons’ infant son, Wilfred.