Labour MP David Lammy has said he would be willing to “pour whatever I can get my hands on” over a white supremacist during a tirade about “hard right” politics.

The MP for Tottenham defended the tone of his attacks on Steve Bannon and Boris Johnson during a Twitter spat with Tim Montgomerie, founder of the ConservativeHome website, on Sunday.

Describing the US political strategist – heard talking about his links with Mr Johnson in a newly-revealed video – as a “hard right, white supremacist”, Mr Lammy also accused the Tory leadership frontrunner candidate of being a “pathological liar” for claiming he had no association with Mr Bannon.

Mr Montgomerie claimed Mr Lammy tweeted “more like Trump than Boris” and said: “You endlessly pour scorn on all of your political opponents, poisoning public discourse in the process.”

The Labour MP responded: “I would pour whatever I can get my hands on, on an avowed White Supremacist. My ancestors died fighting this shit. I don’t give two bloody hoots what your apologist arse thinks about it … You lay down with dogs you get fleas.”

The dispute rekindled the debate about the best way to combat openly racist activists and the rise of far-right. As the online row escalated on Sunday afternoon, Mr Lammy continued to insist there was no polite way to tackle racism and xenophobia.

After Mr Montgomery criticised his tone for a second time, the Tottenham MP responded: “What is it about those that would like to be considered “English Gentlemen” when they are called out for being nothing less than thugs.”

“This isn’t about social media you snake,” he said, addressing the Conservative commentator. “It’s about the Hard Right, White Supremacy and [Boris Johnson]’s fickle pursuit of power whatever the cost.”

He added: “How low will your apologism sink? You think me calling out Boris Johnson is “no better” than him taking advice from Bannon who wears “racist” and “xenophobe” as a “badge of honour”? Are you paid to be Boris’ stooge or just morally bankrupt?”

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Mr Montgomerie replied: “I’m not in the pay of Boris but do add me to your list of racists. As far as I can tell that list covers everyone who ever dares to question your saintliness.

“I hope you will one day come to regret your shouty, shouty approach to public debate but for the moment will say goodbye.”