Federica Mogherini, who replaced Ashton, had given the Briton the post "to facilitate nuclear negotiations with Iran," the European Union said.
Ashton had in her former job led talks between the so-called P5+1 group -- comprising Britain, China, France, Russia and the United States plus Germany -- and Iran.
Ashton's nomination will "ensure the continuity of the negotiations," the EU said.
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At the most recent negotiations in Vienna, Iran and the P5+1 group missed a November 24 deadline for a comprehensive agreement.
But they agreed to keep talking and set new deadlines of March 1 for a broad political framework and June 30 for the fine print of the final deal.
Key differences remain on when sanctions will be lifted on Iran and on what limits will be placed on its enrichment of uranium, a process used to produce energy but which can also lead to the fissile material needed for an atomic bomb.
Iran denies seeking nuclear weapons.
Mogherini, the former Italian foreign minister who took over from Ashton on November 1, will however participate in ministerial-level meetings with Iran and in talks with her Iranian counterpart Mohammad Javad Zarif.