A US judge has ruled that a section of the law banning polygamy violates religious freedom, which could lead to the practice being decriminalised.

Kody Brown – a member of a fundamentalist Mormon group who, along with his family, features in reality TV show 'Sister Wives' – filed a lawsuit against the state in 2011 after he was forced to fee Utah under threat of being charged with bigamy.

Polygamous marriage is illegal in all 50 states in the US, and though formerly practised by the mainstream Mormon Church, it was outlawed in 1890.

Some breakaway groups, however, including the Apostolic United Brethren of which Brown and his four wives are a part, continued the practice. They believe that it brings exaltation in heaven.

Around 38,000 fundamentalist Mormons are thought to practise or believe in polygamy, most of whom live in Utah.

Until now, it has also been illegal in Utah to legally marry one person and live with others who are considered to be spouses – polygamous Mormon men usually legally marry one woman, and then have 'spiritual' unions with other wives – though this law is rarely enforced.

Last December, district judge Clark Waddoups ruled that Utah law prohibiting this cohabitation is a violation of religious freedom, and he upheld this decision on Wednesday.

Brown has released a statement praising Waddoups' ruling, saying it "brings closure for our family and further reaffirms the right of all families to be free from government abuse."

"While we know that many people do not approve of plural families, it is our family and based on our religious beliefs," Brown continued.

"Just as we respect the personal and religious choices of other families, we hope that in time all of our neighbours and fellow citizens will come to respect our own choices as part of this wonderful country of different faiths and beliefs."

Utah Attorney General Sean Reyes has confirmed that his team are reviewing the ruling. It is expected to be appealed.

Republican Governor Gary Herbert said on Thursday that he disagrees with Waddoups' decision and believes state laws regarding polygamy should be upheld.

"I think it's probably not good policy and good practice for families to have that kind of a situation, so that's my own provincial view of traditional marriage," Herbert said.