There are around 8 million Catholics in Vietnam

Father Ly, who was put under house arrest shortly afterwards and arrested in May, was also accused of distributing anti-Communist leaflets.

The priest has been a long time critic of the Vietnamese authorities and had previously spent 10 years in prison for his beliefs.

Vietnamese television showed Father Ly, looking gaunt, listening with his eyes closed to his sentencing by a court in the central city of Hue on Friday.

US Congress has since approved the trade agreement Father Ly was protesting against and President George W Bush signed it into law on Wednesday.

US reaction

But correspondents say his heavy sentence could hasten US Senate action on a separate Vietnam human rights act which Vietnam's Communist Party has severely criticised.

The bill, which the US House of Representatives has already passed, would stop all American non-humanitarian aid to Vietnam unless Hanoi improves its human rights record.

Father Ly's sentencing is only the latest example of recent religious persecution by the Vietnamese Government.

In September, Ho Tan Anh, the 61-year-old leader of the Buddhist Youth Movement, burned himself to death to protest restrictions on his group.

Officials accused Father Ly of threatening public security

A prominent priest of the outlawed Unified Buddhist Church of Vietnam was put under house arrest in June, after he announced plans to escort another church patriarch under detention for medical treatment.

There have also been persistent reports of harassment of minority groups in Vietnam's Central Highlands who belong to unapproved Protestant "house churches."

The Vietnamese Government, which insists its citizens enjoy religious freedom, only allows seven recognised religious organisations.

The Catholic Church is one of them, but followers are strictly controlled by the government, which has the final say on who takes up senior positions in the Church in Vietnam.