Tatts Group shareholders should reject the $11 billion merger with gambling giant Tabcorp because the financial benefits of the proposed deal are "well and truly skewed" in favour of Tabcorp shareholders, says activist investor Sandon Capital.

Sandon will detail its last-ditch bid to try to scupper the long-mooted merger in a letter and presentation to be made public on Friday, in which it will argue Tatts shareholders are giving away at least $1.5 billion in value to Tabcorp if the deal goes ahead as proposed.