MILAN, Oct 27 (Reuters) - London-based investment firm Merlyn Advisors and a former Telecom Italia (TIM) executive on Friday moved to challenge the group's plan to sell its landline grid, proposing an alternative revamp, a letter seen by Reuters showed.

The challenge comes after U.S. fund KKR tabled a binding offer for TIM's grid that valued it at around 23 billion euros, when including debt and some variable components.

The letter, signed by Merlyn Advisors' founder Alessandro Barnaba, an ex-JPMorgan banker, and Stefano Siragusa, a former TIM deputy general manager, is dated Oct. 27 and is addressed to TIM's board of directors.

Under the proposed alternative scheme, TIM would retain its entire fixed network business as well cloud and digital services units, while selling its domestic retail business and its prized Brazilian unit.

Merlyn Advisors represents a group of investors owning in aggregate less than 3% - the regulatory disclosure threshold - of TIM, either through direct ownership or power of attorneys, the letter said.

TIM was not immediately available to comment. (Reporting by Elvira Pollina; Editing by Valentina Za)