By Alan Baldwin

RIO DE JANEIRO (Reuters) - Michael Phelps and Katie Ledecky will both feature in the United States 4 x 100 meters freestyle relay teams at the Rio Olympics, USA Today reported on Monday.

Neither of the American team's best-known swimmers, with 18-times gold medalist Phelps seeking to add to his collection of 22 medals, qualified for the individual event at the national trials.

Five-times Olympian Phelps, the most decorated athlete of all time, is set to compete in three individual events -- the 100 and 200 meters butterfly and 200 individual medley.

The relay line-ups, a closely-guarded secret, do not have to be submitted until the night before the race, but it would be no surprise for the pair to be included in the 4x100m.

Both U.S. relay teams are likely to finish in the top three, which would give Phelps and Ledecky their first medals of the Games.

USA Today, in a report citing an unnamed source with direct knowledge of the U.S. swimming team’s plans, said Phelps would swim in Sunday night's men's relay final.

It said Ledecky, the 19-year-old tipped for gold in the 200, 400 and 800 freestyle, was expected to take part in the preliminary heats on Saturday afternoon in what would be her first competitive dip in the Rio pool.

Swimmers who take part in the preliminary heats, but not the final, also get medals.

It is common for the finals line-up to differ from the preliminaries because it saves the top swimmers for the medal showdown and allows more to share in the glory.

Ledecky finished seventh in the individual 100 freestyle at last month's U.S. trials in Omaha, Nebraska while Phelps did not enter.

Nathan Adrian was fastest in the trials, with Caeleb Dressel second and Ryan Held third. Others in the mix are Anthony Ervin, Jimmy Fiegen and Blake Peroni.

However, Phelps showed impressive speed at the team's pre-Games training camp in Atlanta.

The U.S. swimmers are due to attend a news conference in Rio on Wednesday.

(Editing by Ed Osmond)