Australian duo David Warner and Marcus Stoines withdrew from this summer’s men’s 100 competition.
The pair were scheduled to play for Southern Brave but pulled out due to coronavirus restrictions.
Both are also named to the Australia squad for tours in the West Indies and Bangladesh in July and August.
A spokesperson for the England and Wales Cricket Board (ECB) said: “It is clearly disappointing to lose players of the quality of David and Marcus.”
“But the realities of Covid mean there are practicalities that some players abroad will find difficult to overcome.
“Alternate overseas players will be signed by Southern Brave, and we look forward to The Hundred’s show of world-class cricket this summer.”
Opener Warner, 34, was signed for £100,000 and Stoinis, 31, for £80,000.
The first game of the new 100 Ball competition – a women’s match between Oval Invincibles and Manchester Originals – will air on BBC TV on Wednesday, July 21.
Both the men’s and women’s finals take place at Lourdes on Saturday, August 21.