Liberal Democrat leader Sir Ed Davey has said he will boycott the banquet for Donald Trumps state visit next month, to "send a message" to the US president over the humanitarian crisis in Gaza.
Sir Ed said it went against his "instincts" to decline an invitation from the King, describing this as an "honour".
But he added that Trumps influence over Israel, Qatar and other Gulf states meant "he has the power to stop the horrifying starvation and death in Gaza and get the hostages released".
Trump and his wife Melania will be hosted by King Charles and Queen Camilla for the US presidents second state visit from 17 to 19 September, which includes a banquet in St Georges Hall in Windsor Castle.
Around 150 guests are typically invited to the grand dinners, ranging from politicians and diplomats to religious leaders and celebrities.
As leader of the UKs third biggest party, Sir Ed has previously attended several state banquets, including for French President Emmanuel Macrons visit last month.
Sir Ed said: "Emily and I have spent all summer thinking about this and have prayed about it. There is no honour like an invitation from the King, and not to accept his invitation goes against all of our instincts.
"But I fear we could have a situation where Donald Trump comes to our country, is honoured with a lavish dinner at one of our finest palaces, yet no one reminds him that he has the power to stop the horrifying starvation and death in Gaza and get the hostages released.
"If Donald Trump tells [Israeli Prime Minister] Benjamin Netanyahu to stop this, it ends tomorrow. If Donald Trump uses his influence over Qatar and other Gulf states that Hamas relies on, all the hostages could come home tomorrow.
"Boycotting the banquet is the one way I can send a message to Donald Trump and Keir Starmer that they cant close their eyes and wish this away. We have to speak up, they have to act. Donald Trump must act to end this humanitarian crisis."
Trump has consistently backed Netanyahu during the conflict but acknowledged last month that there was "real starvation" in Gaza, something Israel has denied.
The United Nations has said there is "man-made" famine in Gaza, accusing Israel of restricting the amount of aid entering the territory.
Israel has rejected claims it is responsible for food shortages.
Israels military launched a campaign in Gaza in response to the Hamas-led attack on southern Israel on 7 October 2023, in which about 1,200 people were killed and 251 others were taken hostage.
At least 62,819 people have been killed in Gaza since then, according to the territorys health ministry.
Trump was hosted by the late Queen Elizabeth II for a state visit in 2019, during his first term in office.
Prime Minister Sir Keir Starmer has described the second invitation as "historic" and "unprecedented".
Traditionally, second-term presidents are not offered a state visit and have instead been invited for tea or lunch with the monarch.
While he will be given a full ceremonial welcome, Trump will not get an opportunity to address MPs as the House of Commons breaks up for the annual party conference season the day before he is due to arrive in the UK.
In 2019, then-Lib Dem leader Sir Vince Cable declined to attend Trumps state banquet, saying British politicians should not be "hobnobbing with a man who is on record as a misogynist and a racist".
Then-Labour leader Jeremy Corbyn and Commons Speaker John Bercow were also among those who declined their invites.
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