Chicago ? Subway is joining the increasingly crowded breakfast scramble in a move that the sandwich chain hopes will help add customers and sales.

After years of testing, almost all of Subway’s 23,000 U.S. restaurants will begin selling the meal April 5. When they do, the nation’s largest restaurant chain by number of outlets will be a big player in the breakfast game.

“There are a number of other competitors of ours that are trying to suss out the breakfast opportunity, and I’d rather be in the market before they get there,” said Tony Pace, chief marketing officer at the Subway Franchisee Advertising Fund Trust, the chain’s consumer marketing division. “Is there going to be competition now? Of course. And it’s going to be fierce.”

The menu, already served in some U.S. cities and throughout Canada, sticks with Subway’s sandwich specialty. Featuring customizable “omelet sandwiches,” the options include a combination of eggs or egg whites, cheese, ham, bacon, steak, sausage, peppers and onions in addition to Subway’s other toppings. Sandwiches will be served on English muffins, flatbread or the company’s traditional sub rolls.

While franchise owners — who operate all of the company’s 25,000 North American locations — determine the prices of the breakfast items, suggested prices will range from $1.75 to $6. A combo meal featuring an English muffin sandwich and coffee would be $2.50.

Advertising for the new menu will begin next week.

Breakfast has become a popular addition to fast-food chains in recent years as companies clamor for diners. Since coffee, eggs and other breakfast ingredients often come cheap, the meals typically can rake in big profits. While McDonald’s promotes its new dollar breakfast menu, other competitors are getting into the mix. Among them: Taco Bell and Wendy’s.

It’s not a sure thing. As the economy soured, so did breakfast sales as customers cut back on spending and unemployed workers stopped visiting restaurants. According to research firm NPD Group, the number of customers buying breakfast at fast-food restaurants slipped 2 percent in 2009.

But that’s not keeping restaurant chains from trying. Restaurants added more than 460 new breakfast items to menus in 2009, according to market researcher Mintel. That’s more than in 2008 and 2007.