British Prime Minister Boris Johnson on Thursday made an awkward joke about "chlorinated chickens" while meeting with Vice President Mike Pence in London.

The joke reportedly left the room "visibly befuddled."

Johnson's joke was in reference to the US practice of rinsing meat in chlorine toward the end of the production process to protect consumers from certain pathogens.

Johnson's exchange with Pence comes amid a rough week for the British prime minister, in which he's suffered major defeats in Parliament that have derailed his plans for Brexit.

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British Prime Minister Boris Johnson on Thursday made a joke about chickens while meeting with Vice President Mike Pence at No. 10 Downing Street that left the room "visibly befuddled," The Associated Press reported.

As Pence told Johnson that the US was ready to negotiate a free-trade agreement with the UK, the prime minister responded with enthusiasm but also said he wanted to ensure any deal was one that works for "all sides."

"Mike, it's fantastic to have you here," Johnson said. "And we will drive that free-trade agreement forward. Of course, the US economy is a wonderful, massive opportunity for UK folks."

Johnson went on to express dismay that people in the US "don't eat any British lamb or beef or haggis from Scotland."

"And I know that you guys are pretty tough negotiators," Johnson said. "So we're going to work very hard to make sure that that free-trade deal is one that works for all sides."

Before launching into his joke about chickens, Johnson then added that the UK would not accept any deal that could weaken the state-funded National Health Service.

"We're not too keen on that chlorinated chicken, either," the British prime minister said. "We have a gigantic chlorinated chicken of our own here on the opposition benches."

Read more: What happens now MPs have voted to delay Brexit until 2020?

In addition to using it as a means of insulting his political opponents, Johnson's jocular chicken comment was made in reference to the US practice of rinsing meat with chlorine toward the end of the production process to protect consumers from pathogens such as salmonella and campylobacter.

The topic has come up repeatedly in conversations on a US-UK trade deal. There are concerns that chlorine rinsing permits lower hygiene standards at other points in the production process. In March, US Ambassador to the UK Woody Johnson dismissed such concerns as "inflammatory and misleading."

Boris Johnson's exchange with Pence comes amid a rough week for the British prime minister, in which he's suffered major defeats in Parliament that have derailed his plans for Brexit and even seen his own brother abandon him and resign from the UK government over the prime minister's agenda.