Tory party staff have reportedly been given one week to hand over all information they have about Boris Johnson’s Downing Street flat renovation – and could face criminal charges if they do not comply. The Electoral Commission is investigating claims that Conservatives did not register donations that were used to partly fund refurbishments to the flat Johnson shares with his fiancee and child above 11 Downing Street. Officials were emailed on Thursday night by the party’s human resources team to warn them that failure to hand over any communications to the inquiry was a criminal offence, the Times and the Sun reported. The email was sent from Alan Mabbutt, a senior official and registered legal officer, and said materials must be handed over by 7 May. “You are put on notice that this is a criminal investigation,” the email said. It warned that staff who “knowingly falsify, conceal, destroy or otherwise dispose of information” … “could be committing a criminal offence of perverting the course of justice”. Separate inquiries into how the refurbishment was funded are also being carried out by the head of the civil service, Simon Case, and the independent adviser on ministers’ interests, Lord Geidt. Reports have claimed that Johnson was given a loan of £58,000 from a Conservative donor to cover the costs of the decoration. Johnson has said that he “personally” paid for the renovations but has refused to be drawn on whether he initially covered the costs with a donation. He said the row over the funding was a “farrago of nonsense” and that “I don’t think there’s anything to see here”. But announcing the investigation into the funding on Wednesday, the Electoral Commission said there were “reasonable grounds” to suspect multiple offences may have been committed. The watchdog said it would be investigating “whether any transactions relating to the works at 11 Downing Street” could have violated donation reporting requirements. It has the power to issue “disclosure notices”, which force individuals and organisations to hand over documents and information. According to a YouGov poll, the affair does not seem to have damaged the party’s reputation, with the Conservatives narrowly extending their lead over Labour this week despite just 14% of respondents saying they had not heard about the issue. The politics professor and author Tim Bale said although opinion polls suggested the wider electorate were not overly concerned by the inquiry, any potential wrongdoing still had serious consequences. “The opinion polls suggest that it’s not affecting the Conservatives poll rating at the moment however that is a different issue from whether actually something occurred here which shouldn’t have occurred,” he told Sky News on Friday. “If it was the case that someone else initially paid for this refurbishment, however trivial that may seem to some people, that that is an issue because it should have been declared. We do have rules for a reason.”