Britain's governing Conservative Party has been caught up in another damaging public row following comments made by former Foreign Secretary Boris Johnson about women wearing Muslim full-face veils known as burkas.

Johnson, who resigned from prime minister Theresa May's cabinet in July over her proposals for Britain's departure from the European Union, is one of the party's most outspoken figures, and is seen by many as future party leader.

In a column in the Daily Telegraph newspaper, he likened women wearing burkas to "bank robbers" and said "it is absolutely ridiculous that people should choose to go around looking like letter boxes".

There has been widespread criticism of his remarks from leading party figures. May said "it's not language I would have used, and I think it was wrong to use that language", which had "clearly caused offence", while former party chairwoman Baroness Warsi, the first Muslim woman to sit in a British cabinet, called for disciplinary action against Johnson if he did not apologize.

The chairman of the Conservative Muslim Forum, Mohammed Amin, also criticized the article, saying it would "whip up hatred", adding "Boris Johnson is a master of the English language－he must understand exactly what effect his language will have".

But sources close to Johnson insist he stands by his comments, with one telling the BBC: "We must not fall into the trap of shutting down the debate on difficult issues. We have to call it out. If we fail to speak up for liberal values then we are simply yielding ground to reactionaries and extremists."

Johnson's justification for his remarks is that the burka is "oppressive", but he said he opposed a full ban－as is the case in several European countries－as this would be interpreted by radicals as "a clash of civilizations" between Islam and the West, and could lead to "a general crackdown on any public symbols of religious affiliation".

This latest Conservative Party dispute adds to pressure on prime minister May, who has already lost Johnson and David Davis, her Secretary of State for Exiting the European Union, from the cabinet this summer over her handling of Brexit.

On his recent visit to the United Kingdom, US president Donald Trump talked up the qualities of Johnson, a former Mayor of London, as a potential future prime minister.