Boris Johnson has stood by his remarks about the burka after the Conservative Party chairman told him to apologise.

The former foreign secretary has been criticised for saying Muslim women wearing burkas “look like letter boxes” and comparing them to “bank robbers”.

His remarks have been branded “offensive” and “deliberately provocative”.

But a source close to Mr Johnson said: “It is ridiculous that these views are being attacked.”

“We must not fall into the trap of shutting down the debate on difficult issues,” the source added.

“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.”

Mr Johnson’s remarks, in a Daily Telegraph article, have provoked criticism from Muslim groups, some Tory MPs and opposition parties.