Former U.S. senator Rick Santorum won the non-binding Republican presidential primary in Missouri, television networks projected, in the first result from the three state nominating contests being held on Tuesday.

Santorum had a big lead over front-runner Mitt Romney with about one-third of the votes counted when CNN and MSNBC projected him as the winner.

It was the second victory for the former U.S. senator from Pennsylvania, who narrowly beat Romney to win Iowa's caucuses on January 3. Missouri is one of three states holding Republican presidential nominating contests on Tuesday, with Minnesota and Colorado holding caucuses. They are the latest contests in the state-by-state battle for the Republican nomination to face President Barack Obama, a Democrat, in the November 6 U.S. election.

The Missouri primary is considered a beauty contest because the candidates do not win delegates who will take part in the August Republican convention where the party's presidential nominee will officially be chosen. Missouri Republicans will select convention delegates in caucuses on March 17 in the state's two-step process.

Santorum is battling with Newt Gingrich to be the top conservative alternative to the more moderate Romney.

(Reporting by Deborah Charles)