An office manager and an account manager were also made redundant.
Porter joined Sputnik last September after former MD Lucy Hackett and three other senior staff members left to form Marmalade PR.
The latest staff cuts come hot on the heels of the redundancies of three junior employees in January. They also follow the loss of key client Argos after parent company Home Retail Group decided to consolidate its brands into Iris PR (PRWeek, 27 March).
Agency co-founder Kate Waddington said Sputnik was now down to 12 employees.