SEOUL (Reuters) -South Korea's financial regulators and stock bourse operator have prepared measures to curb unfair trading practices, including illegal short-selling, after President Lee Jae Myung ordered a "one-strike out" policy for violators.

A joint task force of the Financial Services Commission (FSC), the Financial Supervisory Service and the Korea Exchange will target stock price manipulation, the FSC said on Wednesday, vowing to recover illicit gains and expel offenders from capital markets in the long run.

"The highest level of fines (100% of short-sale orders) will be imposed on serious violations of short-selling, with business suspension and trading restrictions," the FSC said.

In March, the country's financial regulator said it concluded a probe into short-selling breaches, imposing fines on a total of 13 foreign banks.

Naked short-selling, or selling stocks without borrowing them first or determining if they can be borrowed, is banned under South Korea's Capital Markets Act.

The Asian nation lifted its ban on short-selling for the first time in five years in March, after preparing a system to better detect illegal trades.

The FSC plans to gradually tighten listing criteria, such as market capitalisation and revenue, for "the stock market's growth and credibility".

(Reporting by Jihoon Lee; Editing by Sumana Nandy)