The eight men's teams in the ECB's new 100-ball competition have until January to negotiate with players they wish to retain from the squad they selected in October 2019, while in the women's tournament, players can opt to roll their contracts over to 2021 or negotiate with other teams if they wish to do so.
In the men's competition, the Phoenix have retained Zampa alongside three locally-based players: Warwickshire's Henry Brookes and Adam Hose, and Worcestershire seamer Pat Brown. In the women's tournament, Gardner is joined by three players contracted to Central Sparks in the regional domestic structure in Ria Fackrell, Evelyn Jones and Marie Kelly.
Zampa is the second overseas player to commit to the men's Hundred for 2021, following Trent Rockets, while Gardner follows international team-mate Elyse Villani, West Indies' Stafanie Taylor and Deandra Dottin, and New Zealand's Sophie Devine, who will be her captain at the Phoenix.
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"I'm over the moon to know that I'm staying with Birmingham Phoenix and to help this new team mount a serious challenge in this first year of the Hundred," Zampa said.
"Birmingham is a fantastic city and I'm really looking forward to calling Edgbaston home. Hopefully we can welcome crowds and our new fans to the ground because, having played against England at Edgbaston, I know that the atmosphere there is awesome."
"We've got the makings of an excellent Birmingham Phoenix squad and I can't wait to get going in next year's competition and to continue working with head coach Ben Sawyer, who I know so well from Sydney Sixers and the Australia women's team," Gardner said.
"Speaking to players around the world, there's so much excitement around the Hundred and it's fantastic to see so many of the best players coming for this first competition. I believe that we've got a great chance of going all of the way and bringing the trophy back to Birmingham."
Meanwhile, the Hundred has added a new sponsor, with online car retailer Cazoo becoming its 'principal partner'.
Birmingham Phoenix squads so far:
Men's: Moeen Ali (captain), Henry Brookes, Pat Brown, Adam Hose, Dom Sibley, Chris Woakes, Adam Zampa
Women's: Sophie Devine (captain), Ria Fackrell, Ashleigh Gardner, Amy Jones, Evelyn Jones, Marie Kelly