Outgoing Liberal Democrat leader Tim Farron claimed the Prime Minister should be “ashamed of herself” for putting trade links with the oil rich state ahead of public security.
Mr Farron’s hard-hitting remarks came after the PM said the Government is taking advice on what is able to be made public from the probe.
Mrs May’s predecessor David Cameron agreed to an inquiry into foreign funding of Islamist extremists in 2015 as part of a deal with Lib Dems in return for their support on the extension of air strikes into Syria.
Reports have suggested the findings may never be published because of the sensitivity of the investigation’s information regarding Saudi Arabia.
Mr Farron told the Press Association: “It is a scandal that the Government are suppressing this report.
“The only conclusion you can draw is that they are worried about what it actually says.
“We hear regularly about the Saudi arms deals, or ministers going to Riyadh to kowtow before their royal family, but yet, our Government won’t release a report that will clearly criticise Saudi Arabia.
“All this Government seems to care about is cosying up to one of the most extreme, nasty and oppressive regimes in the world.
“You would think our security would be more important, but it appears not.
“For that Theresa May should be ashamed of herself.”
In a written answer to a parliamentary question, the PM said: “The review into the funding of Islamist extremism has improved the Government’s understanding of the nature, scale and sources of funding for Islamist extremism in the UK.
“Ministers are considering advice on what is able to be published and will report to Parliament with an update in due course.”