A Pentagon inspector general is launching its own investigation into whether President Donald Trump’s former national security adviser Michael Flynn took payments from foreign entities without approval, according to a letter released by House Democrats.

The inspector general of the Department of Defence is probing whether Mr Flynn had “failed to obtain required approval prior to receiving any emolument from a foreign government”, according to the letter to the House Oversight and Government Reform Committee.

The investigation concerns payments of more than $33,000 that Mr Flynn received from Russian state-backed television network Russia Today in 2015. As a retired Army officer, Mr Flynn was obligated to request permission from the Department of Defence and State Department before receiving payments from foreign governments, members of the Oversight Committee allege.

The Department of Defence told The Independent that they initiated their investigation of Mr Flynn on 4 April – weeks before the Oversight Committee first took their accusations public.

The Oversight Committee released documents on Wednesday showing that the Pentagon warned Mr Flynn not to take foreign government payments without approval. A letter sent by the DIA to Mr Flynn upon his resignation in 2014 explicitly states that “foreign compensation requires advance approval”.

“The Emoluments Clause of the US Constitution ... prohibits receipt of consulting fees, gifts, travel expenses, honoraria, or salary by all retired military personnel, officer and enlisted, regular and reserve, from a foreign government unless congressional consent is first obtained,” the letter reads.

The letter instructs Mr Flynn to “obtain advance approval from the Army” before receiving any such payments.

Trump and Putin passionately kiss in street mural Show all 14 1 /14 Trump and Putin passionately kiss in street mural Trump and Putin passionately kiss in street mural A lesbian couple kisses in front of mural depicting Russian President Vladimir Putin and Republican presidential candidate Donald Trump, on the walls of a barbecue bar 'Keule Ruke' on May 19, 2016 in Vilnius, Lithuania. Barcroft Media/Getty Trump and Putin passionately kiss in street mural A lesbian couple kisses in front of mural depicting Russian President Vladimir Putin and Republican presidential candidate Donald Trump, on the walls of a barbecue bar 'Keule Ruke' on May 19, 2016 in Vilnius, Lithuania. Barcroft Media/Getty Trump and Putin passionately kiss in street mural VILNIUS, LITHUANIA - NOVEMBER 23: A woman walks past a mural showing U.S. President-elect Donald Trump (R) blowing marijuana smoke into the mouth of Russian President Vladimir Putin on the wall of a bar-b-que restaurant on November 23, 2016 in Vilnius, Lithuania. Many people in the three Baltic nations of Lithuania, Latvia and Estonia are concerned that Russia, because Trump has expressed both admiration for Putin and doubt over defending NATO member states, will be emboldened to intervene militarily in the Baltics. Sean Gallup/Getty Trump and Putin passionately kiss in street mural A woman walks past a mural on a restaurant wall depicting US Presidential hopeful Donald Trump and Russian President Vladimir Putin greeting each other with a kiss in the Lithuanian capital Vilnius on May 13, 2016. Kestutis Girnius, associate professor of the Institute of International Relations and Political Science in Vilnius university, told AFP -This graffiti expresses the fear of some Lithuanians that Donald Trump is likely to kowtow to Vladimir Putin and be indifferent to Lithuanias security concerns. Trump has notoriously stated that Putin is a strong leader, and that NATO is obsolete and expensive. Petras Malukas/AFP/Getty Images Trump and Putin passionately kiss in street mural AP Trump and Putin passionately kiss in street mural A child walks past a graffiti depicting Russian President Vladimir Putin, left, and Republican presidential candidate Donald Trump, on the walls of a bar in the old town in Vilnius, Lithuania, Saturday, May 14, 2016. (AP Photo/Mindaugas Kulbis) AP Trump and Putin passionately kiss in street mural People walk past a mural on a restaurant wall depicting US Presidential hopeful Donald Trump and Russian President Vladimir Putin greeting each other with a kiss in the Lithuanian capital Vilnius on May 13, 2016. Kestutis Girnius, associate professor of the Institute of International Relations and Political Science in Vilnius university, told AFP -This graffiti expresses the fear of some Lithuanians that Donald Trump is likely to kowtow to Vladimir Putin and be indifferent to Lithuanias security concerns. Trump has notoriously stated that Putin is a strong leader, and that NATO is obsolete and expensive. Petras Malukas/AFP/Getty Images Trump and Putin passionately kiss in street mural A man photographs a mural on a restaurant wall depicting US Presidential hopeful Donald Trump and Russian President Vladimir Putin greeting each other with a kiss in the Lithuanian capital Vilnius on May 13, 2016. Kestutis Girnius, associate professor of the Institute of International Relations and Political Science in Vilnius university, told AFP -This graffiti expresses the fear of some Lithuanians that Donald Trump is likely to kowtow to Vladimir Putin and be indifferent to Lithuanias security concerns. Trump has notoriously stated that Putin is a strong leader, and that NATO is obsolete and expensive. Petras Malukas/AFP/Getty Images Trump and Putin passionately kiss in street mural AP Trump and Putin passionately kiss in street mural A young woman walks past a mural showing U.S. President-elect Donald Trump (R) blowing marijuana smoke into the mouth of Russian President Vladimir Putin with the slogan "make everything great again," in reference to Trump's campaign slogan of "Make America Great Again," on the wall of a bar-b-que restaurant on November 23, 2016 in Vilnius, Lithuania. Many people in the three Baltic nations of Lithuania, Latvia and Estonia are concerned that Russia, because Trump has expressed both admiration for Putin and doubt over defending NATO member states, will be emboldened to intervene militarily in the Baltics. Sean Gallup/Getty Trump and Putin passionately kiss in street mural A morning commuter stops to look at a mural on a restaurant wall depicting US Presidential hopeful Donald Trump and Russian President Vladimir Putin greeting each other with a kiss in the Lithuanian capital Vilnius on May 13, 2016. Kestutis Girnius, associate professor of the Institute of International Relations and Political Science in Vilnius university, told AFP -This graffiti expresses the fear of some Lithuanians that Donald Trump is likely to kowtow to Vladimir Putin and be indifferent to Lithuanias security concerns. Trump has notoriously stated that Putin is a strong leader, and that NATO is obsolete and expensive. Petras Malukas/AFP/Getty Images Trump and Putin passionately kiss in street mural Restaurant owner Dominykas Ceckauskas pose next to a mural on the wall of his establishment depicting US Presidential hopeful Donald Trump and Russian President Vladimir Putin greeting each other with a kiss in the Lithuanian capital Vilnius on May 13, 2016. Kestutis Girnius, associate professor of the Institute of International Relations and Political Science in Vilnius university, told AFP -This graffiti expresses the fear of some Lithuanians that Donald Trump is likely to kowtow to Vladimir Putin and be indifferent to Lithuanias security concerns. Trump has notoriously stated that Putin is a strong leader, and that NATO is obsolete and expensive. / AFP / Petras Malukas (Photo credit should read PETRAS MALUKAS/AFP/Getty Images) Petras Malukas/AFP/Getty Images Trump and Putin passionately kiss in street mural A passerby photographs a mural showing U.S. President-elect Donald Trump (R) blowing marijuana smoke into the mouth of Russian President Vladimir Putin on the wall of a bar-b-que restaurant on November 23, 2016 in Vilnius, Lithuania. Many people in the three Baltic nations of Lithuania, Latvia and Estonia are concerned that Russia, because Trump has expressed both admiration for Putin and doubt over defending NATO member states, will be emboldened to intervene militarily in the Baltics. Sean Gallup/Getty Trump and Putin passionately kiss in street mural Getty

Mr Flynn’s lawyer said the former lieutenant general reported his plans to visit Russia to the Defense Intelligence Agency (DIA). He also said Mr Flynn provided the DIA with a thumb drive of files, one of which states that Mr Flynn used a speakers bureau for the trip, implying it was a paid engagement. The lawyer, Robert Kelner, is calling for public release of the documents on the thumb drive.

Elijah E Cummings, the Ranking Member of the House Committee on Oversight and Government Reform, argued that Mr Flynn had “concealed the payments he received from foreign sources” and said the documents raised “grave concerns" about why he had done so.

“Our next step is to get the documents we are seeking from the White House so we can complete our investigation,” Mr Cummings said in a statement.

White House Press Secretary Sean Spicer called the investigation "appropriate," and said the administration welcomes the inquiry. But the White House has also declined to turn over any documents concerning Mr Flynn’s work for the Trump transition team. Mr Flynn was fired as national security adviser shortly after Mr Trump took office, before it was revealed that he misled Vice President Mike Pence about his contact with foreign governments during the transition.

White House Press Secretary Sean Spicer claimed on Wednesday that documents pertaining to Mr Flynn’s activities before the inauguration are “not in possession of the White House”. He also dismissed requests for records of Mr Flynn’s contact with foreign governments after Mr Trump took office.

“To ask for every call and contact that a national security officer is made is pretty outlandish,” Mr Spicer said at a press conference.