Platt will be replaced by Peter Long, previously non-executive chairman, who will act as executive chairman until a successor is appointed.
Platt commented: “Since joining in 2014 I have worked hard, within a difficult market, to structure a fragmented business into one set for success. I believe those actions will serve Countrywide well in the future.
“However, the time is now right for me to step aside. I look forward to seeing the business build back and return to growth.”
The latest profit warning was issued on 18 January, with profits expected to drop 22% to £65m in 2017, down from £83.5m in 2016.
Countrywide said that the slumping profits were due to a disappointing performance in the sales and lettings area – particularly in the capital and the South-East.
Compared to 2016 figures, total income from the sales and lettings arm is expected to drop 14% to £360m.
The estate agent, which owns high street brands such as Gascoigne-Pees, Hamptons and Taylors, said that total group income for the full year is expected to be around £672m, down from £737m the year before.
Long said that he plans to return the group to profitable growth, after acknowledging that Countrywide’s core sales and lettings area has “lost focus”, and that a “key priority” will be to implement the necessary changes for the area to start delivering again.
“Working together with this experienced executive team I feel confident we will return the business to profitable growth,” said Long.
Paul Creffield, previously managing director of commercial development, has been appointed group operation director. Creffield’s responsibilities will now include heading the sales and lettings division.
In a statement, Countrywide said that “the board acknowledges the commitment Alison has given to the group and wishes her well for the future”.
At the time of writing, Countrywide’s shares were up 2.20% to 102.20p following news of Platt’s departure.
Further details on the group’s strategy will be given at its preliminary results, due on 8 March 2018.