MILAN, June 5 (Reuters) - Italy's Serie A soccer league has kicked off a new bidding round for the rights to broadcast matches from 2018-21, its chairman said, after a last-ditch attempt by Spain's Mediapro to salvage a previous deal failed.

In February, Serie A accepted an offer by Mediapro, a Chinese-owned group, for the rights to screen almost 400 games over the next three seasons at just above 1.05 billion euros ($1.23 billion).

But the Italian soccer league last month annulled the contract signed with Mediapro after the Spanish group failed to present the necessary bank guarantees.

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Serie A Chairman Gaetano Micciche said on Tuesday guarantees eventually submitted by Mediapro had been deemed "unacceptable in several respects, both formal and quantitative".

Serie A is now due to start meetings with broadcasters interested in the rights, a source close to the league said.

A spokesman for Mediapro said the group would do everything possible to win the rights in order to pursue its plan to set up a Serie A channel to sell to other media groups.

A Milan court in May annulled a tender process that would have allowed Mediapro to re-sell bundles of matches following an appeal by the Italian unit of British broadcaster Sky.

Serie A has been trying for the past year to assign the rights for the next three seasons.

Mediaset Premium, the pay-TV arm of Italy's biggest commercial broadcaster Mediaset, has found it hard to recover the investment it made to buy the rights in the last three years. ($1 = 0.8565 euros) (Reporting by Elvira Pollina, writing by Valentina Za, editing by Ed Osmond)