Reports suggest that the former prince was worried about his security
The former Crown Princess of Nepal, Himani Shah, has left for Singapore with her three children, immigration officials said.
They boarded a Silk Air flight in the afternoon from Kathmandu's Tribhuvan international airport, officials said.
The former Crown Prince of Nepal, Paras Shah, left for Singapore two weeks ago, the first time he had left the country in more than two years.
His relatives have denied that he has left Nepal for good.
"Himani Shah, wife of former Crown Prince Paras Shah, has left Nepal this afternoon along with her three children Purnika, Kritika and Hridayendra," an immigration official told the BBC.
Paras left for Singapore in early July and there has been speculation that he was worried about the safety of the former royal family members in the Himalayan nation.
But his relatives say the couple have left the country for their children's education.
Paras is looking for a good school for his three young children, whose education is being disrupted by constant strikes and traffic problems in Nepal, a relative had earlier told the BBC.