- Video report by ITV News Political Correspondent Daniel Hewitt
Labour MPs urged Mr Johnson to apologise for remarks where he described a parliamentary bill which forces the government to ask for an extension to Brexit talks as the "Surrender Act".
A number of MPs - including both Leave and Remain, Labour and Conservative - revealed they have received death threats, with some highlighting language used by the prime minister.
Mr Johnson said while he deplored threats made to MPs, he added it was important not to "impoverish political debate" by refusing to use certain phrases.
Speaking on a visit to Harlow, Essex, Mr Johnson said: "I think the threats against MPs and particularly female MPs are absolutely appalling and we're doing a lot of work to give MPs the security that they need.
"But then there's another question which is - can you use words like 'surrender' to describe a certain act or a certain bill?
"And, quite frankly, I think that you can and if you say that you can't then you're kind of impoverishing the language and impoverishing political debate because, after all, the use of that kind of metaphor has been going on for hundreds of years."
However, some members of the Brexit Party agree with the PM's rhetoric that has been seen as 'inflammatory' by some MPs in the Commons.
At a Brexit Party rally on Friday, one woman told told ITV News: "'Surrender' is harmless language, and 'bollocks to Brexit' is a bit over the top, but of course they get away with that.
"They get away with calling us 'racist', 'bigots', 'stupid', 'selfish'... We get it all."
Another supporter said: "I just think they're overreacting. They need to stop being babies, grow up!"
"They're words at the end of the day. It's more about the action that they actually take.
"Stop babbling on and actually do something."
The prime minister also reaffirmed the government's intension that the UK will leave the EU by October 31 - despite the Benn Law which requires the government to ask for an extension if a Brexit deal is not agreed.
Asked if he was looking to get around the Benn Law, Mr Johnson said: "No, I must say that we will obey the law but we're confident that we can come out on October 31 and the best way to do that is to get a deal.
- Video report by ITV News Correspondent Angus Walker
"And so that's why the surrender act is so damaging.
"I won't hide it from you, it has had the effect with our European friends of making them think 'hmm, maybe parliament can block this thing, maybe they will be forced to extend'.
"If you're in a negotiation, that obviously makes it more difficult but we are still cautiously optimistic that we can do it."
Mr Johnson's chief of staff Dominic Cummings last night admitted there were loopholes in the legislation.
He said: "There are obviously loopholes in it because the Remain lawyers are babbling about on Twitter... They themselves have said there are loopholes."
When asked if it would be something the government could look to exploit, he said: "I didn't say that, did I?"
Mr Cummings was confronted by Karl Turner MP on Thursday evening, with the Labour politician telling Mr Cummings he had received death threats, to which Mr Cummings replied: "Get Brexit done."
On Wednesday, Mr Johnson dismissed a Labour MP’s complaint that his “inflammatory” language risked provoking attacks as “humbug”.
He was facing calls to apologise for language that pits politicians against voters and was even criticised by his sister Rachel Johnson for using “strongman” tactics.