Jack Dorsey will have to quit his role as chief executive of Square if he wants the full time CEO role at Twitter.
Twitter CEO Dick Costolo is stepping down on 1 July, with Dorsey becoming interim boss, juggling the role with his position at Square.
But in a statement, Twitter's board stresses that the committee set up to find a permanent chief will only consider candidates "who are in a position to make a full-time commitment".
Dorsey is already executive chairman of Twitter and has not publicly commented on whether he wants the CEO gig on a permanent basis, but last week he told Recode that "I'm Square CEO and that won't change".