Catherine Ashton will continue mediating talks between Iran and world powers on Tehran's disputed nuclear program.
Federica Mogherini, who took over for Ashton as EU foreign policy chief on November 1, has appointed Ashton as her special adviser for facilitating the nuclear talks with Iran, the EU announced on December 5.
Iran and six powers - the United States, China, Russia, Britain, France and Germany - failed last month to resolve a 12-year stand-off over Tehran's nuclear ambitions, and gave themselves seven more months to clinch an historic deal.
The six powers want to curb Iran's uranium enrichment further to lengthen the time it would need to build a bomb.
Iran denies it is using its civilian nuclear power program to develop a nuclear weapons capability.