ISLAMABAD, Pakistan — Facing the near certain prospect of being removed from office in a no-confidence vote in a matter of days, Prime Minister Imran Khan of Pakistan refused to resign in a defiant speech on Thursday, doubling down on his allegations of a foreign conspiracy against him and deepening a political crisis that has embroiled the country for weeks.
In a televised speech, Mr. Khan accused the United States of a conspiracy to topple his government by inciting a political campaign to remove him from office. He also declared that he would face the no-confidence vote in Parliament, which is expected on Sunday, despite having lost a parliamentary majority this week.
“I have never accepted defeat,” Mr. Khan, the former international cricket star turned politician, said. “I always fight till the last ball. I want the entire nation to see on that day who sold their conscience.”
For weeks, Pakistan has been gripped by a political crisis after Mr. Khan, 69, appeared to lose support from the country’s powerful military last year and a coalition of opposition parties moved to vote him out of office this month.
The tide appeared to turn against him this week, after some members of his political party defected and several parties in his governing coalition split away — appearing to give the opposition the 177 votes, five more than needed in the 342-member National Assembly to remove him from office.
With his political support slipping away, Mr. Khan has scrambled to keep his coalition intact: He gathered thousands of supporters at a rally in Islamabad on Sunday, replaced the chief minister of Punjab to retain the support of one allied party and repeatedly denounced opponents as part of a foreign conspiracy against him.
In recent days, he has centered his political messaging around a purported letter that contained threats to his government. Officials said the letter was written by a former Pakistani ambassador to the United States, who conveyed threats by a “senior foreign official.”
“We received an official document which we must say is against our people,” Mr. Khan said in the speech on Thursday, adding that the letter warned that “Pakistan can face severe consequences if I survive no confidence.”
At a political rally on Sunday, he displayed a piece of paper that he took from his pocket, which he said was evidence of a foreign conspiracy but did not say by which country. He has revealed few other details about the letter and has not provided a copy to Parliament or to the news media.
In his speech on Thursday, he identified the threatening country as the United States, which has long been a favorite target of his political speeches.
He said that American officials said that Pakistan “will have to suffer” if he remained as prime minister.
“No reason was stated,” he said. “They are treating Pakistan as if we are their slaves.”
American officials dismissed Mr. Khan’s characterization of events.
“There is no truth to these allegations,” a State Department spokeswoman said Thursday, speaking on condition of anonymity in line with diplomatic protocol.
Opposition politicians have cast doubt on the letter’s authenticity and Pakistani security officials have dismissed it as an internal diplomatic communication that Mr. Khan has misrepresented.
The political crisis comes as Pakistan, home to 220 million people and the second-largest Muslim population in the world, wrestles with double-digit inflation that has pushed the cost of basic goods, like food and fuel, beyond many Pakistanis’ reach. The economic challenges have fueled criticism that Mr. Khan has mismanaged the economy and failed to deliver on his touchstone promise of creating an Islamist welfare state.
As dissatisfaction with his government grew, his opponents announced their intention to move for a no-confidence motion in Parliament. Since securing the votes needed to oust him from office, opposition leaders have urged him to resign and have assailed his claims of an international conspiracy as undermining the country’s democratic process.
“He has been advised by some people to call a democratic process an international conspiracy,” said Bilawal Bhutto Zardari, chairman of the opposition Pakistan Peoples Party. “They are only doing it for their own interests — not the country’s interest. This is against the country. This is against our Constitution, democracy and the nation.”
Lawmakers had been expected to begin debate on the no-confidence vote on Thursday evening, but the National Assembly session was adjourned within minutes of opening after a boisterous protest by the opposition when the government tried to delay the debate.
If Mr. Khan is voted out of office, lawmakers will choose an interim prime minister to serve until the next general election, scheduled for 2023. Many analysts expect they will select Shehbaz Sharif, an opposition leader.
Mr. Sharif is the younger brother of former Prime Minister Nawaz Sharif and is the former chief minister of Punjab, the country’s most populous and prosperous province.
In his speech Thursday night, Mr. Khan defended his government’s three-and-a-half year record, claiming that he outperformed his rivals’ past governments, and emphasized the earlier achievements in his life.
“I am a very fortunate person,” he said. “God gave me everything, fame, money.” He said that he entered politics to make Pakistan an independent, self-respecting country.
Mr. Khan also warned that even if he lost the no-confidence vote, he would not let the conspiracy against him succeed, hinting that he may seek to return to Parliament as a member of the opposition.
“The people will neither forget nor forgive you,” he said, referring to his opponents. “Neither will they forgive those who are handling you. The people will always remember that you sold your country. Through a foreign conspiracy, you tried to topple a government that had an independent foreign policy.”
Salman Masood reported from Islamabad, Pakistan, and Christina Goldbaum from Kabul, Afghanistan.
