Sarah Horner is to join Blue Rubicon in October as an associate director. She is currently based in the US, but previously worked closely with Tempest-Hay at Fishburn Hedges, where he was a director.
The move follows Blue Rubicon's appointment of Fishburn Hedges consultant Jenna Frost, who subsequently moved to Australia.
An agency insider said: 'Gordon is recreating Fishburn Hedges at Blue Rubicon. His business is taking on Fishburn Hedges at its own game.'
But Tempest-Hay dismissed the speculation and said: 'We have our own style, approach and culture. We're not trying to recreate anyone else's.'
Since Tempest-Hay joined Blue Rubicon in 2004, the agency has pitched against Fishburn Hedges and won accounts including Cadbury Schweppes' and Unilever UK's corporate accounts.
Fishburn Hedges chairman Neil Hedges conceded that Tempest-Hay may have been influenced by Fishburn Hedges' planning function, but argued that the two agencies were currently 'deuce' at winning accounts.
'The first half of this year has been our most successful,' said Hedges, adding that the agency had secured £2.1m of new business in the period.
Tempest-Hay joined Blue Rubicon after nearly five years at Fishburn Hedges. He has been responsible for client services and new business.
Blue Rubicon has been named PRWeek consultancy of the year for the past two years.
Horner joins the agency after spending two years in Boston while her husband was studying at Harvard. Previously, she worked on Fishburn Hedges accounts such as the Learning & Skills Council.