The heavyweight fight for the state’s most lucrative casino ­license is back on.

After failing to win local support for a casino in Foxborough, Las Vegas casino giant Wynn ­Resorts signed an option Wednesday to buy land on the Mystic River in Everett, known as the former Monsanto Chemical Co. site, where developer Steve Wynn ­intends to build an urban gambling resort on the water, Wynn executives have confirmed.