An investigation into the foreign funding of extremist Islamist groups may never be published, the Home Office has admitted.

The inquiry commissioned by David Cameron, was launched as part of a deal with the Liberal Democrats in December 2015, in exchange for the party supporting the extension of British airstrikes against Isis into Syria.

But although it was due to be published in the spring of 2016, it has not been completed and may never be made public due to its "sensitive" contents.

It is thought to focus on Saudi Arabia, which the UK recently approved £3.5bn worth of arms export licences to.

Theresa May: London terror attack shows Britain too tolerant of extremism

A spokesperson from the Home Office told The Independent a decision on the publication of the report would be taken “after the election by the next government”.

But in a separate interview with The Guardian, a spokesperson said the report may never be published, describing its contents were “very sensitive”.

Tom Brake, the Liberal Democrat foreign affairs spokesman, has written a letter to the Prime Minister pressing her on when the report will be published and what steps she proposes to take to address “one of the root causes of violent extremism in the UK”.

“You will agree with me that the protection of our country, of the British people, is the most important job of any government," he wrote. "Certainly, more important than potential trade deals with questionable regimes, which appear to be the only explanation for your reticence.

“When will this report be finished and published? And what steps do you propose to take to address one of the root causes of violent extremism in the UK?”

Mr Brake accused Ms May of adopting a “short-sighted approach” to the funding of violent Islamist groups in the UK and urged that those who fund them should be called out publicly.

10 examples of Saudi Arabia's human rights abuses Show all 10 1 /10 10 examples of Saudi Arabia's human rights abuses 10 examples of Saudi Arabia's human rights abuses In October 2014, three lawyers, Dr Abdulrahman al-Subaihi, Bander al-Nogaithan and Abdulrahman al-Rumaih , were sentenced to up to eight years in prison for using Twitter to criticize the Ministry of Justice. AFP/Getty Images 10 examples of Saudi Arabia's human rights abuses In March 2015, Yemen’s Sunni President Abd-Rabbu Mansour Hadi was forced into exile after a Shia-led insurgency. A Saudi Arabia-led coalition has responded with air strikes in order to reinstate Mr Hadi. It has since been accused of committing war crimes in the country. Getty Images 10 examples of Saudi Arabia's human rights abuses Women who supported the Women2Drive campaign, launched in 2011 to challenge the ban on women driving vehicles, faced harassment and intimidation by the authorities. The government warned that women drivers would face arrest. Getty Images 10 examples of Saudi Arabia's human rights abuses Members of the Kingdom’s Shia minority, most of whom live in the oil-rich Eastern Province, continue to face discrimination that limits their access to government services and employment. Activists have received death sentences or long prison terms for their alleged participation in protests in 2011 and 2012. Getty Images 10 examples of Saudi Arabia's human rights abuses All public gatherings are prohibited under an order issued by the Interior Ministry in 2011. Those defy the ban face arrest, prosecution and imprisonment on charges such as “inciting people against the authorities”. Getty Images 10 examples of Saudi Arabia's human rights abuses In March 2014, the Interior Ministry stated that authorities had deported over 370,000 foreign migrants and that 18,000 others were in detention. Thousands of workers were returned to Somalia and other states where they were at risk of human rights abuses, with large numbers also returned to Yemen, in order to open more jobs to Saudi Arabians. Many migrants reported that prior to their deportation they had been packed into overcrowded makeshift detention facilities where they received little food and water and were abused by guards. Getty Images 10 examples of Saudi Arabia's human rights abuses The Saudi Arabian authorities continue to deny access to independent human rights organisations like Amnesty International, and they have been known to take punitive action, including through the courts, against activists and family members of victims who contact Amnesty. Getty Images 10 examples of Saudi Arabia's human rights abuses Raif Badawi was sentenced to 1000 lashes and 10 years in prison for using his liberal blog to criticise Saudi Arabia’s clerics. He has already received 50 lashes, which have reportedly left him in poor health. Carsten Koall/Getty Images 10 examples of Saudi Arabia's human rights abuses Dawood al-Marhoon was arrested aged 17 for participating in an anti-government protest. After refusing to spy on his fellow protestors, he was tortured and forced to sign a blank document that would later contain his ‘confession’. At Dawood’s trial, the prosecution requested death by crucifixion while refusing him a lawyer. Getty Images 10 examples of Saudi Arabia's human rights abuses Ali Mohammed al-Nimr was arrested in 2012 aged either 16 or 17 for participating in protests during the Arab spring. His sentence includes beheading and crucifixion. The international community has spoken out against the punishment and has called on Saudi Arabia to stop. He is the nephew of a prominent government dissident. Getty

Accusing the Conservatives of being "worried about upsetting their dodgy friends in the Middle East", he said party had "broken their pledge to investigate funding of violent Islamist groups in the UK".

He added: “That short-sighted approach needs to change. It is critical that these extreme, hard line views are confronted head on, and that those who fund them are called out publicly."

It comes after Home Secretary Amber Rudd suggested during a leadership debate, that UK arms sales to Saudi Arabia are good for industry.

The Government has recently approved £3.5bn worth of arms export licences to Saudi Arabia and a stream of British ministers have visited the kingdom to solicit trade, despite its ongoing involvement in the bombing campaign in Yemen.