The governor of Iowa is being targeted by atheist groups for signing a proclamation that encourages the public to join the Iowa 99 County Bible Reading Marathon that will start at the end of the month.

To be held from June 30 to July 3, the four-day Bible reading marathon will be held in front of all the state's 100 courthouses.

"We will unite in honouring God's Holy Word and celebrating our First Amendment freedoms. We will unify the body of Christ around the Holy Bible. We will re-establish the reading of the Word in every home so that we may hear His Word, know His Word, and be doers of His Word!" declared organiser Kingdom Builder Enterprises.

In his proclamation, Gov. Terry Branstad said reading the Bible regularly will renew "the mind of men, thus transforming the life of the individual which ultimately affects the lives of those in the family, the local community, the state, and the Nation."

He also encourages individuals and families in Iowa to read the Bible daily "until the Lord comes."

But his action has riled up the Freedom From Religion Foundation (FFRF), a group that advocates the separation of State and Church, which is threatening to sue him.

In a letter sent to the Governor on Wednesday, FFRF lawyer Andrew Seidel told him that "by issuing this intensely religious proclamation and encouraging Bible-reading, you send a message that Iowa prefers religion over non-religion, and the Christian religion over any other religion."

Seidel claims that Branstad's proclamation "betrays" the sacrifices made by America's founders who established the principle of separation of Church and State.

In his proclamation, Branstad said, "All Scripture is essential to prepare us to be the people that God wants us to be and to accomplish the purpose for which He created us."

He said America has paid tribute to the Bible for its important influence in its development. The leaders who cited the Bible's role in America's growth included Presidents George Washington, Andrew Jackson, Abraham Lincoln and Woodrow Wilson, he added.

The American Humanist Association (AHA) told Barnstad that as governor, he has the "duty to uphold the Constitution, a duty he clearly ignores by using his office to promote Christianity."

"The governor is inviting litigation by showing a complete disregard for the Establishment Clause, and we urge him to withdraw the proclamation immediately," the AHA said.

Also objecting to the proclamation are the American Civil Liberties Union (ACLU) of Iowa and the Iowa Atheists and Freethinkers.

The FFRF lost in court after it challenged President Barack Obama's 2011 National Day of Prayer speech, according to CBN News.