(Reuters) – Activist investor TCS Capital Management on Tuesday urged Yelp (NYSE:YELP) Inc to either explore a sale or a merger with online-services company Angi Inc, sending shares of the service-recommendation site 10% higher in premarket trading.

Yelp is “shockingly undervalued” and could be sold for more than twice its share price, said TCS Capital, one of the company’s largest investors with a stake of over 4%.

Yelp’s market valuation stood at about $2.1 billion, about $500 million more than Angi’s, as of last close.

TCS Capital founder Eric Semler said a merger with Angi, formerly known as Angie’s List, could create a major player in the $500 billion home services market.

Yelp and Angi compete in the online home services market, helping customers with repairs and renovations and other home care needs. Yelp also provides a platform for user reviews on local businesses from restaurants and dentists to mechanics.

“As a former board member and longtime investor in ANGI, I believe that a Yelp and ANGI combination would yield enormous revenue synergies and cost savings that could ultimately double the value of Yelp’s shares,” Semler said.

Angi did not immediately respond to a Reuters’ request for comment, while a Yelp spokesperson said the company “maintains an active dialogue with our shareholders and values constructive feedback on our business and ways to create value.”

Shares of Yelp were up at $35.70 before the bell. They have risen 47% since going public in early 2012, while the benchmark S&P 500 index has more than tripled in the period.

The company is witnessing strong demand for its advertising products at a time when businesses are cutting back on marketing, with revenue rising 13% in the last reported quarter and Yelp raising its full-year net revenue outlook.