Gordon Brown has criticised the official inquiry into the Greensill affair as unsatisfactory and missing vital evidence that meant the government was not properly scrutinised for its actions.

The former prime minister’s comments come as the widow of Jeremy Heywood, a former top civil servant who was heavily criticised in the inquiry’s report, described the process as a travesty set up to scapegoat her husband and distract attention from events after his death.

Brown said the report was “so unsatisfactory” that he wanted its author, the solicitor Nigel Boardman, to be called to explain publicly why “as a reviewer he did not interview key people, why he did not consider vital evidence from Lady Heywood and others that made clear Jeremy Heywood was implementing government policy decided by ministers”.

He said Boardman should also explain “why he appears to justify the current practice of commercial lobbying for financial gain by ex-ministers as acceptable and thus has no proposals to regulate clear and unjustifiable conflicts of interest”. An investigation should look into how Boardman was appointed as a reviewer “given his associations with those he was investigating”, he said.

Boris Johnson has already been accused of orchestrating a cover-up over the lobbying scandal after an official review mildly rebuked the former prime minister David Cameron.

The report said Heywood was “primarily responsible” for the businessman Lex Greensill securing a role in government during Cameron’s premiership. The former cabinet secretary “should have considered the issue of conflicts of interest”, as it should have been apparent that Greensill was building a company, Boardman found.

In a scathing response on Friday, Suzanne Heywood accused Boardman of repeatedly denying requests for her late husband to have representation after she first approached the review in late April, and only including her a week before publication.

“Last week I was called in and Mr Boardman read out his conclusions to me. I tried to challenge him on his independence, to which he wouldn’t respond, so it has been travesty of process,” she told BBC Radio 4’s Today programme.

“They have been trying to set up my husband as far as I can see to divert attention from things that happened much later after he died. I am horrified that I have to be here to try and defend my husband against what has been a fabricated attack on him and an absolutely horrible process.”

She defended her husband’s original role on the basis that the then coalition government was seeking to prioritise supply-chain finance, with which Greensill was familiar, as a means of helping small- and medium-sized enterprise. Greensill’s appointment was made with ministerial approval and he had come with a clean CV, she said.

The Cabinet Office has said the inquiry process was fair. It said everyone referenced in the report was treated in equivalent terms by the review team and that Lady Heywood had access to all relevant papers.

Boardman was appointed in April to run an independent investigation into government contracts and lobbying involving a number of senior Conservative politicians including Cameron, the chancellor, Rishi Sunak, the MP and former health secretary Matt Hancock and the peer Francis Maude.

After leaving government, Cameron became an adviser to Greensill Capital and lobbied ministers, including Sunak, for access to government-backed loans.

Critics have said Boardman should not have been in charge of the inquiry because of his close relationship with the government and the Conservative party. He has been a non-executive director at the Department for Business, Energy and Industrial Strategy and is a former Tory party candidate.