Soames joined Charles Barker (now merged with WS) in 1971 and became MD of the public affairs division in 1978.
Four MD of public affairs Jeremy Fraser said Soames' arrival would provide the practice with 'more Conservative contacts and experience of how politics ran before we had landslide majorities'.
He added: 'Most of us weren't even working when the Tories were last in power.'
Fraser himself left WS three years ago to establish Four's PA business, which now has 16 staff.
In addition to her role at WS, Soames has been director of
Liberty and a council member of the Hansard Society.