More than 1.5million properties across Britain could be infested with Japanese knotweed, a new analysis shows. 

Experts at invasive plant specialist Environet claimed Japanese knotweed is wiping around £21.4billion from the value of homes across Britain. 

Environet's analysis, conducted with Censuswide, found that 7 per cent of houses in Britain are affected by Japanese knotweed, equating to approximately 1.58million. 

Japanese knotweed typically knocks around 5 per cent off the value of a property, equating to £13,500 on average. 

According to Environet's latest live data, Bristol, Merseyside, Greater Manchester, Greater London and Lancashire are hotspots in Britain for the species. 

The Japanese knotweed growing season is now underway, meaning in locations where it is already present, it is likely to grow rapidly over the coming months, according to Environet. 

On average, there are 1.1 incidents of Japanese knotweed per square mile in England, rising to 3.9 in Wales, Environet said. 

Separate research conducted by Environet with YouGov found that 32 per cent of Britons would not, under any circumstances, buy a property affected by Japanese knotweed. 

But 32 per cent of Britons surveyed said they would consider buying a house affected by Japanese knotweed if they could get a discount on the property and a professional treatment plan was in place. 

Andrew McColl, chairman of the Residential Property Surveyors Association, said: 'Discovering Japanese knotweed can have significant financial implications, as removal and management plans are often costly and may require anywhere from five to ten years to fully implement. 

'By ensuring you have accurate information before exchanging contracts, you can better anticipate and mitigate these expenses and potential complications, particularly those that may arise when dealing with lenders.'

Mortgage lenders will sometimes refuse to loan against homes affected by Japanese knotweed, unless a plan to get rid of it for good is in place, alongside an insurance-backed guarantee. 

Sellers are also legally required to declare the presence of Japanese knotweed at their property. 

Emily Grant, a director at Environet, said: 'For most people, their home is their biggest asset, yet over 1.5million households are seeing their property value undermined by knotweed. 

'Despite the fact knotweed is now much better understood than it was a few years ago, and many people are now quite pragmatic about it, a significant proportion of buyers still won't touch an affected property under any circumstance.'

Japanese knotweed is an invasive plant species that spreads rapidly. 

It was introduced to Britain in the nineteenth century as an ornamental garden plant, but has since become an invasive weed subject to legal control measures. 

According to the Royal Horticultural Society, Japanese knotweed is a persistent, clump-forming herbaceous perennial. In spring, rounded pink buds form at ground level and from these reddish-purple shoots emerge. The soft shoots grow rapidly, becoming bamboo-like stems.

Underground, Japanese knotweed forms a system of roots and rhizomes, and, according to the RHS, it is not unusual for these to extend down 1 metre or more.

Japanese knotweed can, the RHS says, outcompete native plants to space, light, moisture and nutrients. 

Concerningly, it can also cause damage to certain hard surfaces. It can exploit joints, cracks and weaknesses in paving, tarmac and asphalt, allowing shoots to emerge and create uneven surfaces, the RHS said.

In some cases, Japanese knotweed can also degrade or alter the banks of watercourses. 

The RHS said: 'Japanese knotweed often colonises the banks of rivers, streams and ditches; this can contribute to erosion and increase the likelihood of flooding.'

While it is not an offence to have Japanese knotweed growing in your garden, it is an offence to plant it or allow it to grow into the wild, and it must be prevented from spreading.

You should not add Japanese knotweed to your council green waste bin, home compost, or your household waste bin. 

Recent research from Confused.com found that the average cost of removing Japanese knotweed permanently was up to £1,910 on average. 

Matthew Harwood, a home insurance expert at Confused.com, said: 'Home insurance doesn't usually cover the cost of removing Japanese knotweed itself, but you may be covered for damage it causes, depending on your policy and whether you've taken steps to control it. 

'It's important to declare it when taking out insurance, as failing to do so could risk future claims being rejected.'

Since 2013, a seller is required to disclose whether Japanese knotweed is present on their property via a TA6 form.

If a seller fails to disclose the presence of Japanese knotweed at their property but knew of its existence, a buyer could make a legal claim for misrepresentation against the seller after the sale.  

Some lenders are still cautious about Japanese knotweed, and may not offer a loan on a home where it is present. 

But a growing number of lenders will be happy with an insurance-backed treatment plan in place. The seller will need to arrange the eradication plan before the sale is completed. 

There is no industry-wide standard when it comes to Japanese knotweed and each lender has its own lending criteria. 

Buyers should always get a house survey completed and ask the surveyor to take a view on the Japanese knotweed situation.