By Yilei Sun and Julie Zhu

BEIJING/HONG KONG (Reuters) - Alibaba shares slumped 9% to their lowest since June on Monday, as the firm's upsized $10 billion buyback programme failed to ease concerns about a regulatory crackdown on co-founder Jack Ma's e-commerce and financial empire.

A sharp sell-off over two sessions has knocked almost $116 billion off the tech giant's Hong Kong-listed shares.

The downward spiral intensified when Chinese regulators announced on Thursday the launch of an antitrust investigation into Alibaba and said they would summon its Ant Group affiliate to meet. Alibaba's U.S. shares sank more than 15% during the day.

"The antitrust investigation into Alibaba has yet to specify the penalties, which is worrying investors a lot," said Zhang Zihua, chief investment officer of Beijing Yunyi Asset, adding a probe outcome could "greatly change" the company valuations.

Putting investors more on edge was news over the weekend that China's central bank had asked Ant to shake up its lending and other consumer finance operations.

These developments are part of a crackdown on monopolistic behaviour in China's booming internet space in general, but Ma's business empire in particular after he publicly criticized the regulatory system for stifling innovation.

Last month, Chinese regulators abruptly suspended Ant's blockbuster $37 billion initial public offering in Shanghai and Hong Kong, which was on track to be the world's largest, just two days before its planned debut.

"The new regulations are hurting big internet platforms, so you see Tencent and other tech companies are also seeing their share prices going down," said Li Chengdong, a Beijing-based tech analyst.

"Alibaba now is the target of the regulators so the reaction is stronger."

Regulators have warned Alibaba about the so-called "choosing one from two" practice under which merchants are forced to sign exclusive cooperation pacts preventing them from offering products on rival platforms.

The State Administration for Market Regulation said on Thursday that it had launched a probe into the practice.

The gloom due to the regulatory crackdown overshadowed Alibaba's decision, announced on Sunday, to raise its share repurchase programme to $10 billion from $6 billion, effective for a two-year period through the end of 2022.

Alibaba shares could trade lower in the near term due to the "regulatory overhang", Nomura said in a note on Monday.

But the cheaper value will be attractive for long-term investors, Nomura added as it kept a "buy" rating on Alibaba's U.S.-listed stock and retained a target price of $361. The stock closed at $222 on Thursday.