Evan McMullin says: "It's never too late to do the right thing"
A former CIA operative has announced he is to run against Donald Trump in the race for the US presidency.
Evan McMullin is reportedly being put forward as a third-party conservative alternative by Republicans disillusioned with their party's candidate for November's White House election.
He most recently served as chief policy director for the US House of Representatives' Republican Conference.
He has worked in Congress since 2013, according to a LinkedIn profile.
The House Republican Conference said in a statement he is no longer employed there.
Writing on Twitter, Mr McMullin says he is "standing up to run for president because it's never too late to do the right thing."
Like Mr Trump, the 40-year-old Mormon has never held elected office.
Mr McMullin previously spent 11 years as an operations officer for the Central Intelligence Agency, according to reports.
He said in a statement to ABC News: "It's never too late to do the right thing, and America deserves much better than either Donald Trump or Hillary Clinton can offer us.
"I humbly offer myself as a leader who can give millions of disaffected Americans a conservative choice for President."
Mr McMullin had been a fierce critic of Mr Trump on social media, saying that opposing the businessman is about "putting principle over power".
"Authoritarians like @realDonaldTrump use promises of law & order to justify infringing on civil rights as they consolidate control by force," he posted on July 21st.
Mr McMullin is expected to file papers with the Federal Election Commission later on Monday.
His candidacy underlines how the 'Never Trump' diehards are still trying to derail the campaign of their own party's standard-bearer, barely three months before voting day.
If Mr McMullin does throw his hat in the ring he would stand little chance of replacing President Barack Obama in the White House.
But he could peel away crucial votes from Mr Trump in swing states, theoretically helping Democratic candidate Hillary Clinton to win the election.
The third-party candidacy could also backfire, rallying anti-establishment Republicans behind Mr Trump.