Millions of homeowners who have been climbing the property ladder since 1998 could find themselves unable to “trade up” by the end of this downturn, new figures show.

Brokers are urging those who have bought in the past decade to take urgent steps to bolster the equity in their homes as the housing downturn threatens to strip hundreds of thousands of pounds in value from properties in the next two years.

Exclusive research by estate agents Savills for The Sunday Times has charted the journey of homeowners at various stages over the past 25 years — looking at those who started with a new-build flat and bought larger properties at five-year intervals.

The results show that while most homeowners will still have sizeable equity in their homes this year, the picture changes dramatically as the downturn gathers pace. The research assumes prices fall 25% from peak to trough.

Most surprisingly, those who bought in 1998 are perilously close to the mortgage “danger zone”, which brokers say applies to anyone with less than 25% equity in their home.

This group will have £74,762 or exactly 25% equity in their home by 2010, tumbling from 51% this year.

Those who bought at the top of the market in 2003 will be deep in negative equity, owing 115% of the value of their home — a huge fall from the 17% equity they hold now.

Lucian Cook of Savills said: “Those who bought since 2003 will suffer greater pain, being left with much the same feeling as those who first bought in 1988 — whose housing cost them substantially more than those who remained in the rented sector for longer, but who also suffered the curse of negative equity for a five-year period from 1990 to 1995.”

Most economists believe that house prices will fall 25% from peak to trough while some are even predicting prices could drop as much as 35%.

According to the Halifax house-price index, prices have fallen nearly 15% in the past year.

Homeowners who bought in the 1980s and 1990s are more insulated than those who bought recently — for example, someone who bought in 1988 will suffer an 11% drop in equity compared with a 22% fall for those who bought in 1993.

However, the precarious position for those who bought in 1998 reveals the extent of the downturn.