The party published a statement online blaming the failure of their party to win a seat on the “crowded” nature of the election and “gormless liberals”.
In the post, Deputy Leader Jayda Franson said they could not gauge their performance in “heavily Muslim areas” because there are “hardly any Britons left in those areas”.
She added: “In areas that are heavily Muslim, multicultural or “trendy”, the Britain First vote collapsed, naturally.
“Unfortunately these areas make up a majority of the new, colonised Islamised-multicultural London.
“In areas that are heavily Muslim, multicultural or “trendy”, the Britain First vote collapsed, naturally”
Jayda Franson, Deputy Leader
“We were in effect swimming against the tide.”
The statement was met with disapproval online, with critics branding the party “sore losers”.
And the anger was fuelled by their mayoral candidate Paul Golding’s decision to turn his back on Sadiq Khan during his acceptance speech – an act which the party called a “dignified protest”.
Facebook user Andrew Macdonald responded to the post, saying: “Britain First fans hear this, London turned their backs on you long before Golding turned his to Khan. Utterly pathetic.”
It now appears that these comments have been deleted from the statement on Britain First’s Facebook page.
While Michael Sajid said: “A great day for London and the whole of the UK. Proud of Mr Khan. You’s are all a bunch of sore losers Britain first.”
Paul Golding was branded a “bigot” for refusing to face the new Labour mayor and turning back around when runner-up Tory candidate Zac Goldsmith took the podium.
Mr Golding defended his behaviour and denied that it was a racist protest.
He said: “Racist? I didn’t turn my back around because he’s Asian, I turned my back on him because he’s a vile man.
“The extreme left lunatic fringe of Labour have taken over with Corbyn.”
Mr Golding won less than 3% of the total votes Mr Khan won, prompting Twitter users to call him a “rejected racist”.
The Britain First candidate has previously claimed that parts of London were being “colonised” by Muslims.
He also said: “All of the British people have all moved out of London and immigrants and Muslims have all moved in. Whole swathes of the city now are completely colonised.”
Former Labour candidate Gareth Siddorn said on Twitter: “I stood against Paul Golding in Sevenoaks in 2010. He was a small-minded, pathetic scumbag then and he remains one now.”
Mr Khan thanked his “amazing” family said his bus driver dad would have been proud of him.
He said: “This election was not without controversy. But I’m proud that London has chosen hope over fear and unity over division.
“I hope that we will never be offered such a stark choice again.”
He added: “The politics of fear is simply not welcome in our city.”