Former Trump campaign aide Rick Gates admitted to an extramarital affair and to falsifying documents during an explosive day of testimony at the bank fraud trial of his mentor Paul Manafort.

Having been brought to the stand by the prosecution for a second day of testimony Mr Gates testified that Mr Manafort instructed him not to tell their firm’s bookkeeper about payments from accounts in Cyprus that held millions of dollars in earnings from consulting work for pro-Russian politicians in Ukraine, with “hundreds of emails” showing that Mr Manafort had approved payments out of the account.

Mr Gates, as the government’s star witness, is crucial to the prosecution’s case. Mr Manafort, 69, has pleaded not guilty to 18 counts of bank fraud, tax fraud and failing to disclose foreign bank accounts containing tens of millions of dollars earned from work in Ukraine.

The charges in federal court in Alexandria, Virginia, largely pre-date the five months Manafort spent on Trump’s campaign as the now-president’s campaign manager. Mr Gates also spent time with the campaign.

Mr Gates’ testimony was part of the prosecution’s effort to prove that Mr Manafort was responsible for financial manoeuvrings that he and other witnesses have testified include filing false tax returns and failing to report foreign bank accounts.

For the defence, it is crucial to undermine Mr Gates’ credibility – with the core of their case being that Mr Gates was in charge of the finances for Mr Manafort’s consulting firm and that Mr Manafort had just been too trusting.

The charges against Mr Manafort stem from US Special Counsel Robert Mueller’s investigation into Russian meddling in the 2016 US election and possible collusion with Trump campaign officials, and are the first such charges to go to trial. Although Mr Manafort’s charges are not related to collusion.

Mr Gates has taken a plea deal with Mr Mueller’s office, having pleaded guilty to lying to investigators and conspiring to defraud the United States. Mr Manafort’s sought to attack that first.

Under questioning from defence lawyer Kevin Downing Mr gates testified that he provided false information to investigators before he pleaded guilty to charges in February.

“Have they confronted you with so many lies that you can’t remember?” Mr Downing asked Mr Gates.

Mr Gates admitted that he had “another relationship” involving first-class flights and luxury hotels, but denied these were funded with money embezzled from Mr Manafort.

Mr Downing went on to say: “There’s another life, right, the other Rick Gates? The secret Rick Gates?”

Mr Gates, who is 46 and married, admitted: “There was a period in my life, about 10 years ago, when I had a relationship, yes.”

Mr Downing pressed: “As part of your secret life did you maintain a flat – is that what they call it – in London?”

Gates acknowledged that he did for two months and that he took first class flights and stayed in “fancy hotels” around Europe. But said he maintained it by inflating expense reports.

“In essence, I was living beyond my means,” Mr Gates testified. “I’m taking responsibility for it. I made a mistake.”

Mr Gates testified on Monday that he helped Mr Manafort doctor financial statements, hide foreign income and evade hundreds of thousands of dollars in US income taxes. On Tuesday, he acknowledged writing a fraudulent letter to prospective investors in a movie project and possibly submitting personal expenses to Donald Trump’s inaugural committee for reimbursement.

Mr Downing sought to portray Gates as an inveterate liar. “After all the lies you told you expect this jury to believe you?” Downing asked Gates, who responded that he did.

”I’m here to tell the truth,” Mr Gates shot back. “Mr. Manafort had the same path. I’m here.”