Shares in Britain’s biggest estate agent, Countrywide Properties, plunged 25% as it issued its fourth profit warning in eight months and called on shareholders to raise fresh funds to cut its debt.

Countrywide, the company behind Hamptons, Bairstow Eves, Taylors and Gascoigne-Pees, has been hit hard by a downturn in the housing market in London and the south-east, as well as growing competition from new online firms.

The shares fell to a new low of 59p after the warning – four years ago they were trading at more than 600p.

“The market in the first half has continued to be subdued and we have experienced longer transaction cycles,” the group said. “As previously announced, the group entered 2018 with our sales pipeline significantly below that of 2017.”

Countrywide said adjusted profits in the first half would be about £20m lower than a year ago and the shortfall would not be made up in the second half of the year.

The firm is forecasting adjusted earnings before interest, taxation, deprecation and amortisation (Ebitda) of £8.1m for the first half, and £44.7m for the full year – below City forecasts of £51.4m. Last year, it made profits of £67.4m in what it described as a disappointing year, against £83.5m in 2016.

Countrywide wants to cut its debts of £192m by at least half through raising extra capital. It said its largest shareholder, the private equity firm Oaktree, and its lenders were backing its plans.

The Countrywide chief executive, Alison Platt, resigned days after the January profit warning and Peter Long took the reins as executive chairman. He announced a “back to basics” recovery plan in March, which included cutting the head office by a third, taking the firm’s overall headcount from 400 to 350.

The firm moved too far in integrating its lettings and sales businesses. Long has reversed this and has called for senior figures in the branches who left during the Platt years to come back. The register of properties available for sale is broadly back to 2017 levels – up 9% since the start of the year.

Demand in the property market has been hit by higher stamp duty and Britain’s vote to leave the EU two years ago. House prices have been falling in London and parts of southern England, but are still rising elsewhere in the UK.

Henry Pryor, an independent luxury property buying agent, tweeted: