The adviser charged with leading US President Donald Trump's post-election legal challenges, David Bossie, has tested positive for Covid-19, a source familiar with the matter told Reuters.

Bossie, a prominent conservative activist who leads advocacy group Citizens United, tested positive on Sunday, joining White House Chief of Staff Mark Meadows and Housing Secretary Ben Carson as victims of the latest coronavirus outbreak to touch the White House.

Bossie was picked to lead the legal challenges to Trump's election loss.