The Forestry Commission has said the UK faces "an unprecedented level of threat" from exotic pests and diseases.
Joan Webber, the principal pathologist at Forest Research, the Forestry Commission's research body, said many of the new diseases and pests are associated with the international trade in live plants.
"There are protections in place but the EU plant health regime is no longer fit for purpose," she added.
"Too many pests and diseases are still getting through."
The warning comes after the government put in place restrictions on the importation of ash trees into Great Britain to combat chalara dieback of ash.
The deadly disease has already affected trees in England and Scotland, and killed trees in parts of mainland Europe.
More than 100,000 ash trees have already been felled to try to prevent the disease spreading.
The Campaign to Protect Rural England has welcomed the ban.
"We hope this will reduce the spread of ash dieback disease and prevent the loss of this iconic species," said Emma Marrington, CPRE's Rural Policy Campaigner.
"If the disease were to spread across the English countryside then it would have a huge impact on our familiar rural landscapes, including ash trees in hedgerows.
"While positive steps have been taken to control the disease, which was recently confirmed at 30 sites across the UK, CPRE believes that the Government can go further to improve the future for trees and woodlands by:
"Setting up an emergency summit bringing together representatives from all areas of forestry, plant health and conservation to plot a way forward to control ash dieback disease and other threats to our native trees and ensuring that Government scientists give urgent and clear advice to all woodland owners on how to manage the disease.
"Adopting the recommendations in the Independent Panel on Forestry’s final report, with an urgent focus on delivery of the Tree Health and Plant Biosecurity Action Plan by additional investment in research on tree and woodland diseases, resilience and biosecurity controls.
"Supporting the Independent Panel on Forestry’s call to increase England’s woodland cover by 50 per cent by 2060, in order to increase the resilience of our woodlands. CPRE suggests that Government could initiate a competition on where to create a second National Forest for England, following in the footsteps of the successful Midlands National Forest, or make woodland creation a priority for the next round of Nature Improvement Areas.
"Considering extra protections for some of our most important woodlands. For example, CPRE has been campaigning for the Forest of Dean to be made an Area of Outstanding Natural Beauty (AONB). This would ensure that this unique and much-loved area gets the stronger protection it needs and deserves.
"Consulting on improvements to the Hedgerows Regulations so that a broader range of countryside hedges will be retained, if they are threatened with removal. Ash is the second most common hedgerow tree so the more hedges that are classed as ‘important’ under the Regulations, the greater the chance we have of saving more native ash hedgerow trees."