She has sat on the council since 2010, previously holding the justice ministry post.
Sommaruga took over the presidency from Finance Minister Ueli Maurer of the right-wing Swiss People's Party (SVP).
Under Switzerland's so-called “magic formula” in place since 1959, cabinet positions are shared between the four largest parties: the SVP, the Socialists, the right-leaning Free Democratic Party (PLR) and centrist Christian Democrats.
The Green Party made historic gains at federal elections in October, prompting calls that it now deserved a cabinet seat, but it has not yet managed to secure a place in the executive branch.
EXPLAINED: What you need to know about Switzerland's Federal Council
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