The US president is a "threat to our world order" and does not deserve a state visit, Emily Thornberry has said.
The shadow foreign secretary said Donald Trump was a "disgrace to his office" after he announced the US would pull out of the international Arms Trade Treaty.
More than 100 countries, including Britain, have signed up to the treaty which regulates international trade in conventional weapons, including small arms, battle tanks, combat aircraft and warships.
As a result, Ms Thornberry said Trump was not a "leader of the free world", and opposed his upcoming state visit to the UK, expected to take place in early June.
Her comments follow Labour leader Jeremy Corbyn turning down an invitation to a state banquet which will be held for the US president during his visit.
"Maintaining an important relationship with the United States does not require the pomp and ceremony of a state visit," Corbyn said, before criticising the Prime Minster for "kowtowing" to a president who "rips up vital international treaties".
However, foreign secretary Jeremy Hunt said the country should offer "the best possible welcome" to Trump, with the Prime Minister reportedly "looking forward" tothe visit.