Donald Trump’s Jewish son-in-law and key adviser has been caught up in an escalating scandal over the US president’s links to Russia, after it was revealed that he set up a secret “back channel” before Trump even took office.

Jared Kushner, an Orthodox Jew whose wife Ivanka is one of Trump’s five children, is a real estate and media tycoon who was appointed senior White House adviser, with a wide-ranging remit which includes the Israeli-Palestinian conflict.

Until this week, Kushner had largely escaped the focus on Trump’s links with Vladimir Putin’s Moscow regime, which has so far forced the resignation of Trump’s national security adviser and the recusal of his attorney-general from the FBI investigation into Russian influence.

Last week, however, the Washington Post reported that in December last year Kushner sought to set up a secret “back channel” between Trump’s team and Russia.

Adam Schiff, the ranking Democrat on the House Intelligence Committee, said the reports were “obviously very concerning,” adding that – if proven – Kushner’s top-secret security clearance should be removed immediately.

In a familiar refrain, Trump tweeted that it was “fake news,” but he is now under considerable pressure, with several investigations under way into his team’s links with Russia, which is accused of hacking Democratic Party emails in order to help Trump win the White House.

Former director of national intelligence James Clapper said Kushner’s links with Moscow “certainly arouses your concern about what is going on, given Russia… is our primary adversary”.