MILAN (Reuters) - Telecom Italia's (TIM) biggest investor, Vivendi, asked an Italian judge for further documentation to be provided in a case challenging the planned sale of the phone group's landline grid to U.S. fund KKR, a lawyer for the French company said on Tuesday.

Worth up to 22 billion euros ($23.9 billion) and backed by the Italian government, the proposed sale is a key plank of TIM Chief Executive Pietro Labriola's strategy to revamp the debt-laden telecoms company.

($1 = 0.9200 euros)

(Reporting by Elvira Pollina; Editing by David Goodman)