The price of a couple's Hertfordshire house has dropped by more than £250,000 because Japanese knotweed has invaded it, according to an independent surveyor.
With its value falling from an estimated £305,000 to £50,000, experts have told owners Matthew Jones and Sue Banks from Broxbourne that, unless action is taken, it will be impossible to sell.
They have been told 10ft (3m) of soil needs to be removed from beneath the foundations to remove the plant.
The invasive weed was discovered in the garden of their new-build house in April 2009 after they had been living there for about a month.
A couple of months later it was found growing in the dining room.
Japanese knotweed, or Fallopia japonica, was introduced into Britain by the Victorians as an ornamental plant.
In its growing season of April to October it can grow 4in (10cm) a day and solid structures such as brick walls are no barrier.
Mr Jones, 38, explained that he first discovered the plant outside one evening after it had made its way into his garden from a nearby field.
"I was out in the garden and I noticed some stems coming through the lawn," he said.
"They were like asparagus tips but they had a reddish tinge to them. I had never seen anything like that before so I didn't touch it, I went to bed and in the morning it had grown a couple of inches."
Broxbourne Borough Council sent an environmental specialist along who identified Japanese knotweed straight away and advised the couple to contact a solicitor immediately.
Just two months later it had forced its way into the house through the flooring and skirting boards.
Experts have advised that demolishing the house and removing the soil will provide a permanent eradication.
Flamstead gardening expert Matthew Biggs said it was a "major problem".
He said: "By law you are not allowed to move the rhizome [horizontal stems found underground] so if it's dug up from one area it has to be destroyed under controlled conditions or delivered to a site where they have the facilities to get rid of it."
Another option is to use a strong herbicide such as glyphosate, but it can take a long time.
"Even then, on a small clump it can take up to three years of regular treatment to get rid of it," said Mr Biggs.
The pair have eight-month-old twins and Mr Jones said that with a young family they "don't want the hassle" of spraying the plants with strong chemicals a number of times a year, which may not even work.
He said that they now just want to move on and get on with their lives somewhere else: "We've had enough disruption to our life, to get something resolved here could take a number of years and we're not prepared for that."
His fiancee Ms Banks said: "This was supposed to be our dream home, we were so excited about moving in and starting our life together."
The couple are currently suing Roberts of Macclesfield, who handled the purchase of the house, claiming that the solicitors failed to ensure there was a National House-Building Council warranty which provides insurance cover for ten years.
A statement from Mills and Reeves solicitors read: "We act for Roberts & Co in the claim being brought against them by Mr Matthew Jones. The claim is currently subject to proceedings and Roberts & Co deny any liability. As an on-going client matter, we are unable to provide further details."