Both were left out of Ineos Grenadiers team for this year's Tour after struggling for form.
Froome is focussed on the Vuelta de Espana for a third time at the end of October, while Thomas will ride the Giro d'Italia, which starts on Oct. 3.
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Fellow Briton Mark Cavendish will also take part in the eight-stage Tirreno-Adriatico, which starts on Sept. 7.
Cavendish was left off the Bahrain-McLaren team for the Tour, depriving him of the chance to add to his 30 stage wins.
Froome and Thomas will be in good company on the Adriatico with another former Tour winner, Italy's Vincenzo Nibali, set to ride for his Trek Segafredo team.
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