Romania's former King Michael has stripped his only grandson of his royal title, a move that has stunned Romanians.

Michael said in a statement that he had withdrawn the royal titles and privileges from the popular Prince Nicholas, known for his dedication to ecology, cycling and literacy.

The king's statement said the royal family should be led by someone who is "modest, well-balanced and with moral principles", sparking speculation, bewilderment and disappointment.

The popular Nicholas, 30, who was third in line to the throne after his aunt and his mother, has lived in Romania for five years and is unmarried.

In April, he cycled from the northern region of Transylvania to the Black Sea port of Constanta in a 1,000-kilometre journey to raise money for children's issues.

Faced with the cryptic statement, Romanians speculated that a jealous relative had sought to edge Nicholas out of the succession.

His popularity has grown since the surprise announcement and his Facebook page has 23,000 likes. Reports say he has left Romania.

In real terms, the title means little, as Romania is a republic. But Romanians associate their royal family with elegance and dignity that they say their politicians lack. The head of the royal house looks after the family's properties and bestows honours.

In a statement, Nicholas thanked people for his support.

"The royal life means leading my life in a way I find hard to accept," he said. "For this reason, I accept with a lot of pain in my heart the decision of His Majesty King Michael for me."

Michael, 93, abdicated in 1947 when the communists came to power, and settled in Switzerland.

He was banned from Romania during communism, but authorities finally allowed him to visit in 1992.

The statement, released Monday, was signed on August 1. Europe's royal families have been informed.

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