Google is set to launch a property dimension to its UK mapping system.
The new service will allow both estate agents and private sellers to put their property as an overlay on Google Maps.
The plans were outlined at a conference called Estate Agency Events last week, although Google has declined to give official confirmation.
Shares in the property portal Rightmove fell more than 10% as news emerged, the sharpest faller in the FTSE 350 index of companies for the day.
The new service is expected to launch next year and would be similar to a service Google launched in Australia.
That site allows estate agents to list properties for free, with pictures taken from its Street View service and listing details on a map.
Speaking to BBC News, Edward Mead - sales director for Douglas & Gordon estate agents - said that the new system would be a win-win situation for both Google and estate agents.
"The technology to do this is already in place and estate agents are a little busier these days, although transactions are still fifty per cent down on what they once were.
"So this service, which is free, will appeal to estate agents' cost-cutting nature and given that sixty per cent of agents are one-off traders, this will have serious appeal."
Mr Mead said that Google's head of property and classified team, Ben Wood, briefed 30 of England's top estate agents at Estate Agency Events last week, telling them everything about the system, other than an official launch date.
'Hurt estate agents'
But Sarah Beeny, who presents Channel 4's Property Ladder and also runs her own home sales property site Tepilo, told BBC News that the service could well damage estate agents in the long run.
"It will hurt estate agents and it will hurt property sites like Rightmove.
"If it does what Google says it will, then it brings the buyer and seller closer together and that could mean removing blocks in the way, and that could mean no longer having to pay extortionate fees to estate agents.
"It will certainly blow Rightmove out of the water. You can only get your property listed on that site if you are an estate agent - what Google will do is level the playing field and they are doing it for free," she said.
The news shook traders on the London Stock Exchange. At one point, shares in online property portal Rightmove fell by 13% over concern about competition from the world's biggest search engine, although a late afternoon rally saw them close 10% down at £4.95 a share.
The firm remained bullish, despite the news.
Speaking to BBC News, the company's commercial director, Miles Shipside, said his business was still strong and the site was still getting lots of traffic.
"It remains to be seen what actually happens," he said.
"Google is a big name, but they don't always manage to follow things through on a local level.
"We only list property with estate agents due to UK legislation. Agents offer very good value and charge very competitive rates compared to the rest of the world.
Source: BBC News