Jeremy Corbyn has said he will not attend the state banquet at Buckingham Palace in honour of Donald Trump.
The Labour leader argued it would be wrong to “roll out the red carpet” for the US president, whom he accused of using “racist and misogynist rhetoric”.
The US-UK relationship did not need “the pomp and ceremony” of June’s state visit, he added.
Prime Minister Theresa May promised Mr Trump the honour after he was elected in 2016.
Commons Speaker John Bercow and Liberal Democrat leader Sir Vince Cable have already declined to attend the dinner.
In a statement, Mr Corbyn said: “Theresa May should not be rolling out the red carpet for a state visit to honour a president who rips up vital international treaties, backs climate change denial and uses racist and misogynist rhetoric.
“Maintaining an important relationship with the United States does not require the pomp and ceremony of a state visit. It is disappointing that the prime minister has again opted to kowtow to this US administration.
“I would welcome a meeting with President Trump to discuss all matters of interest.”
A spokeswoman for Mr Bercow, who has been critical of Mr Trump’s record in office, said he had been “invited to the banquet, but he will not be attending”.
The SNP’s Westminster leader Ian Blackford is also boycotting the meal, saying Mrs May “should instead be holding meetings to challenge the US administration and raise key issues”.
But Foreign Secretary Jeremy Hunt has said the UK should offer “the best possible welcome” to the president.
And Mrs May’s spokesman said the prime minister was “looking forward to welcoming the president here to build on our special relationship”.
The banquet is scheduled to take place on the first evening of the state visit, which will last from 3 to 5 June.
About 150 guests are expected to be invited, including political leaders and other public figures with cultural, diplomatic and economic links to the US.
During their visit, the president and First Lady Melania Trump will be guests of the Queen and attend a ceremony in Portsmouth to mark 75 years since the D-Day landings.
Mr Trump will also have official talks with the prime minister at Downing Street, although it is not yet clear whether he will address Parliament – as predecessors Barack Obama and Bill Clinton did – amid opposition from many MPs to the idea.
Last July, Mr Trump’s first visit to the UK since he became president in 2017 led to huge protests. He met the Queen and Mrs May hosted a banquet for him at Blenheim Palace.