Shares in Britain’s biggest estate agent, Countrywide Properties, have plunged 25% after 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, a botched revamp of the business and growing competition from new online firms such as Purplebricks.

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. In 2017 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 by raising extra capital. It said its largest shareholder, the private equity firm Oaktree, and its lenders were backing the plans.

Peter Long leads the company as executive chairman after the chief executive, Alison Platt, who had run the firm since 2014, resigned days after the January profit warning. He announced a “back to basics” recovery plan in March, which included cutting head office staff numbers by a third, the equivalent of 150 jobs.

Many senior staff have left in recent years as a result of Countrywide’s decision to shut 200 branches and merge its lettings and sales businesses, hitting its mid-market London business in particular. Under Platt, the group was centralised and branch managers lost the autonomy to recruit, to market locally and to price to win business.

Long, the former boss of Europe’s largest travel firm Tui, is seeking to reverse these moves and reshape the group by empowering regional and branch managers. He has called for senior branch staff 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.

Countrywide said there was no update on the search for a new chief executive. Long has been criticised for the number of boardroom positions he holds. As well as running Countrywide, he is deputy chairman of Tui, and chairman of Royal Mail and Spanish leisure park operator Parques Reunidos Servicios Centrales. He is also president of the Family Holiday Association.

Anthony Codling, an analyst at Jefferies, believes Countrywide may seek to raise about £125m from investors to cut debt and fund the turnaround plan.

“The underlying second-hand housing market is dreadful,” he said. “Even the online challengers are finding that they cannot outperform the current market conditions, and there is little to be done when homeowners stop moving.”