House of Commons Speaker John Bercow would be "strongly opposed" to US President Donald Trump addressing the Houses of Parliament during his state visit to the UK, he has said.
Mr Bercow told MPs that "opposition to racism and sexism" were "hugely important considerations".
Labour and the SNP praised him but critics said he should stay neutral.
President Trump was invited to make a state visit after meeting Theresa May in Washington last month.
A petition to withdraw the invitation - and another one backing the visit - will be debated by MPs later this month.
Responding to a point of order in the Commons, Mr Bercow set out his opposition to a Parliamentary address as part of the state visit.
He told MPs that addressing the Lords and the Commons was "an earned honour", not an "automatic right".
He said he was one of three "key-holders" for Westminster Hall, and referred to the US president's controversial travel ban.
"Before the imposition of the migrant ban, I would myself have been strongly opposed to an address by President Trump in Westminster Hall," he said.
"After the imposition of the migrant ban I am even more strongly opposed to an address by President Trump in Westminster Hall."
The Speaker said he would also be involved in any invitation to address Parliament's Royal Gallery.
'An unprecedented rebuke'
Eleanor Garnier, BBC political correspondent
It was an unprecedented and extraordinary rebuke.
A diplomatic snub that in effect means President Trump will not be invited to address MPs in Parliament.
John Bercow's comments were applauded by MPs on the opposition benches - but critics have said he's abused his position and spoken out of turn.
Mr Bercow's decision risks undermining the prime minister's very public effort to create a new special relationship with the Trump administration.
He added: "I would not wish to issue an invitation to President Trump to speak in the Royal Gallery.
"We value our relationship with the United States. If a state visit takes place, that is way beyond and above the pay grade of the Speaker.
"However, as far as this place is concerned, I feel very strongly that our opposition to racism and sexism and our support for equality before the law and an independent judiciary are hugely important considerations in the House of Commons."
Mr Bercow said the other "key holders" were the Speaker of the House of Lords, Lord Fowler, and the Lord Great Chamberlain, a hereditary peer in charge of certain parts of the Palace of Westminster.
A House of Lords spokeswoman said: "The Lord Speaker was not consulted by Mr Bercow on his statement.
"The Lord Speaker will make his own statement tomorrow to the Lords."
As Speaker, Mr Bercow is the highest authority of the House of Commons and despite having been elected as a Conservative MP, must remain politically impartial.
He is in charge of maintaining order in the Commons and calling MPs to speak.
Other leaders' speeches
- International leaders are sometimes invited to address both Houses of Parliament when they visit the UK
- Recent examples include Colombian President Juan Manuel Santo last year, Chinese President Xi Jinping in 2015 and German Chancellor Angela Merkel in 2014
- Mr Trump's predecessor, Barack Obama, made a speech in Westminster Hall in 2011
The intervention was welcomed by Labour leader Jeremy Corbyn, who has called for the state visit to be postponed, while Lib Dem leader Tim Farron said Mr Trump was "not welcome".
But former UKIP leader Nigel Farage said Mr Bercow had "abused his position" and that to have expressed his opinions in the way he did "devalues this great office".
Prime Minister Theresa May, who has criticised the president's travel ban affecting people from seven mainly Muslim countries, has defended the decision to invite him to make a state visit.
An address to Parliament has not been formally proposed, and no date has been set for the visit.
Downing Street said: "We look forward to welcoming the president to the UK later this year.
"The dates and arrangements for the state visit will be worked out in due course."