Jun Kwang-Woo, the head of South Korea's Financial Services Commission (FSC), suggested to reporters yesterday that it would not approve the deal until the bank provides it with further documentation.
HSBC has been in discussions to buy a 51 per cent stake the Korea Exchange Bank (KEB) for months, offering a price of $6 billion.
Mr Jun said: "It will not be easy to approve the deal in September because we are awaiting additional documents from HSBC and there are more steps we have to go through.''
A report from news source Edaily had suggested that the regulator would clear the deal later this month.
The KEB stake is currently owned by Lone Star Funds.
Previously, the FSC has said that the deal would be approved at an "appropriate time".
HSBC, based in London, is Europe's largest bank.