The next phase of a high-stakes inquiry into whether Boris Johnson misled parliament over Partygate faces being delayed as a result of government failures to provide crucial evidence to MPs, sources have told the Guardian. Despite a range of documents – including the former prime minister’s diaries, event email invites, No 10 entry logs, briefing papers and WhatsApp messages – being requested more than three months ago, some have not been handed over yet. No 10 and the Cabinet Office have been blamed for the delay and MPs on the privileges committee are still waiting to sift through all the evidence before beginning their oral evidence sessions. Johnson is expected to be called to give evidence and the committee initially hoped to summon its first witnesses as early as the end of October. Nevertheless, the timetable has slipped, sources said. They claimed that while there were still plans for oral evidence sessions to be held “before the end of autumn”, work had progressed slower than expected. After some difficulty finding a Conservative MP to fill a vacancy on the committee, Charles Walker was installed on it last month. But delays in receiving vital documents from the government are said to have further hindered the process. A source said it was “impossible to carry out” interviews before all the evidence had been received, so it could be scrutinised and follow-up questions prepared to ask witnesses. Another said the government was being “fucking difficult”. While some information has been provided to the committee, it is believed to have been redacted to such an extent that crucial details are missing and repeated attempts to extract more details have so far proved fruitless. Concerns have been raised that the government could be in contempt of parliament by not satisfying the committee’s request. The initial request for evidence was made to Johnson and the cabinet secretary, Simon Case. It included the Covid status of No 10 staff, deleted documents and number of civil servants disciplined over Partygate. Documents were only requested relating to specific events – including the infamous “bring your own booze” garden party to which 100 Downing Street staff were invited on 20 May 2020 during lockdown, and Johnson’s birthday celebration, for which he received a fine. Government insiders said there was no set date when they were obliged to respond to ad hoc requests for information or documents from a select committee. A document including details of the nearly £130,000 contract for legal advice disparaging the Partygate inquiry was also heavily redacted. Information about the list of people authorised to instruct the legal firm that won the contract, Peters and Peters Solicitors LLP, were withheld in the newly published document. Angela Rayner, Labour’s deputy leader and shadow Cabinet Office minister, said the government should “come clean about the taxpayers’ money wasted on contracting legal advice for Johnson’s law-breaking”. She said despite the promise of the new prime minister, Rishi Sunak, to restore integrity to the government, the Conservatives were “still up to the same old dirty tricks of cover-up and distortion” and accused them of using public cash “with reckless abandon”. A minister signalled the government would continue footing the bill to defend Johnson’s conduct. In a statement quietly slipped out responding to a written question, the then Cabinet Office minister, Chris Philp, said “former ministers may be supported with legal representation after they have left office when matters relate to their time” in government. He argued that the Partygate inquiry “has potential implications for all future statements by ministers of the crown in current and future administrations” and added: “The government has a direct interest in these matters.” Sunak’s spokesperson said on Tuesday: “We do continue to take our responsibilities to assist the committee seriously. We are in correspondence with them directly and responding to their letters, to their requests. That will continue.”