Wendy's/Arby's Group plans to sell its struggling Arby's roast beef sandwich chain to focus on the Wendy's hamburger chain, which its top shareholder expects to be the company's engine of growth.

Its shares were up 27 cents, or 6 percent, at $4.75 in premarket trading.

Sales at Arby's North American restaurants open at least 15 months fell 5.9 percent in its third fiscal quarter, following a 7.4 percent decline in the second quarter.

Nelson Peltz, the company's chairman and largest shareholder, said in a statement that Wendy's is the fast food company's key driver of momentum. Peltz' Trian Fund Management LP owns 18.3 percent of Wendy's/Arby's shares, according to Thomson Reuters data.

Executives said in November they asked Peltz to make a quick decision on a takeover inquiry it received about Wendy's/Arby's in June so the company could resume share buybacks.

Wendy's and Arby's merged in 2008 in a $2 billion deal. They are both trailing behind McDonald's Corp , which has used its large size, Dollar Menu and popular beverage expansion to pummel rivals.

Wendy's/Arby's said UBS Investment is assisting in the sales process.

(Reporting by Phil Wahba in New York and Nivedita Bhattacharjee in Bangalore; Editing by Derek Caney)