The head of the conservative lobbying group the Family Research Council (FRC), has lambasted Archbishop of Canterbury Justin Welby for having 'absolutely no grasp on the biblical foundations of his faith' but 'criticising Americans who do'.

Justin Welby said on Monday that he couldn't understand Christians who supported Donald Trump – provoking consternation from Tony Perkins, a vocal and influential evangelical Trump supporter. He challenged the Anglican leader in his Tuesday 'Washington Update' on the FRC's website, writing: 'Britain's archbishop of Canterbury is considered a "top religious authority" – but after his latest interview, few understand why.

'The leader of the Church of England has absolutely no grasp on the biblical foundations of his faith, yet feels quite free criticizing Americans who do...he lashed out at US evangelicals for their enthusiastic support of President Trump -- seemingly clueless of all the administration has done to advance religious liberty and expression in America.'

Welby had spoken candidly about his bafflement at the majority of US evangelicals (81 per cent in the 2016 election) backing Trump in ITV'S Peston programme on Sunday morning.

'There's two things going through my mind: do I say what I think, or do I say what I should say? And I'm going to say what I think,' he said.

'No, I don't understand it. I really genuinely do not understand where that is coming from.'

Perkins hit back that 'the feeling is mutual', saying Bible-believing evangelicals were 'just as baffled by Welby's approval of same-sex marriage'. Welby had previously told Alastair Campbell that he couldn't 'give a straight answer' on whether homosexuality was sinful, and said same-sex marriage as a secular institution was 'great'. Lambeth Palace later reaffirmed the Church of England's official opposition to same-sex marriage according to Canon law.

Perkins suggested that Welby 'doesn't understand Christian orthodoxy', and praised Trump in contrast.

'Listen to Trump's campaign promises, and then look at his administration's actions. He's pursuing policies on life and religious freedom that are more consistent with biblical principles than any president in decades,' he wrote.

'In all honesty, it's a sad commentary on the state of the church when politicians take positions that are more in line with biblical truth than its so-called leaders.'

Welby challenged Trump again this week after the President retweeted anti-Muslim videos from the far-right political group Britain First. He said it was 'deeply disturbing' that Trump was amplifying extremist voices and called for him to remove the tweets and 'make clear his opposition to racism and hatred in all forms'.