Square, the mobile payment startup that counts former U.S. Treasury Secretary Lawrence Summers on its board of directors, has a new big-name backer: British billionaire Richard Branson.

Branson has made a multimillion dollar investment in Square, according to a Square spokeswoman who declined to disclose the exact amount.

Branson's investment in Square follows a $100 million investment in June led by venture capital firm Kleiner Perkins Caufield & Byers and an investment for an undisclosed amount by credit card company Visa in April.

Square's technology lets merchants accept payments using tablet PCs or mobile phones. The company recorded $10 million in daily transactions for the first time this past weekend, spokeswoman Katie Baynes told Reuters, up from the $4 million per day in gross payment volume that it announced during the summer.

Square has said it is on track to process $2 billion worth of transactions on an annual basis.

The two-year-old company, led by Twitter co-founder Jack Dorsey, faces deep-pocketed competitors eager for a slice of the nascent mobile payment business. Search giant Google Inc recently introduced a digital wallet that lets people use their smartphones to pay for clothing, food and other items.

Branson's investment in Square is a personal investment, said Baynes.

Branson is the founder and highly public face of the Virgin Group, which has created more than 300 businesses that range from air travel to mobile phones.

Virgin Money, the group's financial services arm, recently submitted a bid proposal to acquire the nationalized British bank Northern Rock, sources have told Reuters.

(Reporting by Alexei Oreskovic; Editing by Gary Hill)