The archbishop of Saint Paul and Minneapolis has written an open letter asking Catholics in the US state of Minnesota to vote in favour of a pending constitutional ban on equal marriage.

Bishop John Clayton Nienstedt’s letter was published in the Archdiocese of St Paul and Minneapolis’ official publication, the Catholic Spirit. It said: “Our effort to support God’s unchanging plan for marriage is not a campaign against anyone.

“But rather a positive effort to promote the truth about marriage as a union between one man and one woman.”

Bishop Nienstedt goes on to suggest that equal marriage possess a threat to families, saying: “Kids do need a mom and dad. The well-being of children lies in the balance of a man-made social experiment.

“At the core of this latter concept is the belief that gender doesn’t really matter.”

He ends with a rallying call for Catholics in the state to defeat the campaign for equal marriage:

“We must ensure that Minnesotans know what is at stake and have the correct information about why they should vote ‘Yes’ for the marriage amendment [to ban same-sex marriage]”.

Bishop Niensted adds: “Remember that if you leave the ballot box blank, the government votes ‘No’ for you!”

The Minn Post reports that Catholic churches and groups in the state have already donated $1 million (£629,000) to the campaign which supports the constitutional amendment to define marriage as between one man and one woman.

In 2008, Pope Benedict XVI appointed Mr Niensted as Archbishop of the Archdiocese of Saint Paul and Minneapolis.