The inquiry is thought to focus on British ally Saudi Arabia, which has repeatedly been highlighted by European leaders as a funding source for Islamist extremists, and may prove politically and legally sensitive, the Guardian reports.
The UK has close ties with Saudi Arabia. Prime Minister Theresa May visited the country earlier this year.
In January 2016, a specialist Home Office unit was directed by Downing Street to investigate sources of overseas funding of extremist groups in the UK. The findings were to be shown to Cameron’s then-Home Secretary May.
Eighteen months later, however, the Home Office told the Guardian the report had not been completed and would not necessarily be published, calling the contents “very sensitive.”
A decision on the future of the investigation would be taken “after the election by the next government,” a spokesperson said.
Cameron was urged to launch an investigation in December 2015 as part of a deal with the Liberal Democrats in exchange for the party supporting the extension of British airstrikes against Islamic State (IS, formerly ISIS/ISIL) from Iraq into Syria.
According to the Guardian, Tom Brake, the Lib Dem foreign affairs spokesperson, has written to the prime minister asking her to confirm that the investigation will not be shelved.
“As home secretary at the time, your department was one of those reading the report. Eighteen months later, and following two horrific terrorist attacks by British-born citizens, that report still remains incomplete and unpublished,” Brake wrote.
“It is no secret that Saudi Arabia in particular provides funding to hundreds of mosques in the UK, espousing a very hard line Wahhabist interpretation of Islam. It is often in these institutions that British extremism takes root.”
Lib Dem leader Tim Farron said he felt the government had not held up its side of the bargain.
The report must be published when it is completed, he said, even if its contents are sensitive.
“That short-sighted approach needs to change. It is critical that these extreme, hardline views are confronted head on, and that those who fund them are called out publicly.
“If the Conservatives are serious about stopping terrorism on our shores, they must stop stalling and reopen investigation into foreign funding of violent extremism in the UK.”