David Cameron has been urged to ensure Saudi Arabia and other Gulf states halt the flow of funds to Islamic State if Britain is to join air strikes against the extremists in Syria.
Former Liberal Democrat leader Lord Ashdown expressed concern that the "closeness" between the Conservative Party and rich individuals in the Gulf meant the Prime Minister was reluctant to exert pressure on governments in the region, even though they were supposed to be allies.
He highlighted the way he said that Mr Cameron had shelved a report on the funding of the Muslim Brotherhood after it came up with findings that were "unhelpful" to the Saudis.
The Prime Minister is due to present his comprehensive strategy for tackling IS - including extending RAF air strikes to targets in Syria - to Parliament on Thursday.
Lord Ashdown said it was essential that it addressed the role of the Gulf states such as Saudi Arabia and Qatar.
While both countries were supposedly members of the US-led coalition against IS, he said that Saudi warplanes had not been involved in action for three months while the Qataris had not flown against the extremists for almost a year.
At the same time he said that funds were continuing to flow from the two countries to the extremists.
"I don't say the governments have been doing it, but their rich businessmen certainly have," he told the BBC Radio 4 Today programme.
"The failure to put pressure on the Gulf states - and especially Saudi and Qatar - first of all to stop funding the Salafists and the Wahhabists, secondly to play a large part in this campaign, and other actions where the Government has refused to have a proper inquiry into the funding of jihadism in Britain, leads me to worry about the closeness between the Conservative Party and rich Arab Gulf individuals."
Lord Ashdown pointed to the failure of the Government to publish the report into the funding of the Muslim Brotherhood - even though, he said, it was originally commissioned at the behest of the Saudis.
"Some time ago the Prime Minister, I understand in a single phone call almost off the top of his head, agreed to fund an inquiry into the Muslim Brotherhood on behalf of the Saudi royal family," he said.
"That didn't find what the Saudis wanted it to find - that the Muslim Brotherhood is an extremist organisation. That report has never been published because it came to a conclusion unhelpful to the Saudis."
He said that the Prime Minister's strategy needed to include international pressure on the Gulf states to ensure they "play their part" in the coalition.
"One element of this which is pressure on the Gulf states to stop funding Sunni jihadism, and pressure on the Gulf states if we are going to send our aircraft in to make sure that theirs are present too, is a crucial part of this strategy," he said.
"I ask the question - will the Prime Minister now ensure that in the strategy that he presents on Thursday, that is part of the strategy? Will he now launch an inquiry into the funding of jihadism? If not, I think we are entitled to ask some questions why not."