By Laura Zuckerman

SALMON, Idaho (Reuters) - Idaho's governor said on Thursday he signed into law a measure banning abortions after 20 weeks of pregnancy, making Idaho the third state to enact such a late term abortion prohibition.

Speaking to reporters in Boise, the state capital, Governor Butch Otter, a Republican in his second term, said approving the measure was "the right thing to do."

"I am right-to-life, I have been a right-to-life candidate in government all my life," said Otter, who signed the bill on Wednesday with no advance notice and no fanfare.

The bill, which easily cleared the Republican-dominated Idaho legislature earlier this month, is linked to controversial medical research suggesting the unborn feel pain starting at 20 weeks of fetal development.

The new law and similar proposals under consideration in more than a dozen states, are modeled after a 2010 Nebraska law which has not been challenged in court.

Kansas Governor Sam Brownback, a Republican who took office in January, signed a bill on Wednesday banning abortions performed after 21 weeks in his state. He also signed a bill that will require doctors get a parent's notarized signature for girls aged 17 or younger to get an abortion.

Such measures represent a drive by abortion foes like the National Right to Life Committee to place new curbs on abortion by taking their campaigns to statehouses, where November elections handed sweeping gains to conservative Republicans.

Republican opposition in the U.S. Congress in Washington to funding of Planned Parenthood, a leader in abortion rights and a major provider of abortions, nearly derailed agreement on a key budget deal moving through Congress this week.

The new Idaho law makes it a felony to perform an abortion after 20 weeks unless there is proof the pregnancy endangers the woman's life.

It also opens abortion providers who violate the ban to lawsuits brought by the woman and the unborn's father. The law also authorizes spouses, relatives and other doctors to file legal injunctions against violators.

Sponsors say the restriction is needed in Idaho even though state figures show that about 1 percent of abortions performed there in 2009 involved pregnancies at 20 weeks or more.

Abortion-rights supporters say such measures constitute attacks on Roe v. Wade, the 1973 U.S. Supreme Court decision legalizing abortion.

(Editing by Steve Gorman and Greg McCune)