The trust says its analysis of route proposals for Phase 2a and 2b of HS2 shows that these will impact a minimum of 24 "irreplaceable" ancient woods, of which 11 woods are threatened with direct loss while a further 13 are close enough to suffer damaging secondary effects including noise, dust and lighting.
The figure could rise further, as other woods that could be ancient but do not appear yet on Natural England's Ancient Woodland Inventory have also been identified – some by HS2 Ltd, others by the Woodland Trust.
Woodland Trust chief executive Beccy Speight said: "Any loss or damage to ancient woodland is a disaster for the natural environment, particularly when you consider how little we have left. For large infrastructure projects like HS2 to be riding roughshod over them, rather than setting an example to avoid them, is totally unacceptable.
"HS2 Ltd will say it's planting millions of trees along the route – that's all well and good, but no amount of new trees can ever recreate ancient woodland."
The Environmental Impact Assessment (EIA) for Phase 2a, which will extend the high-speed network from the West Midlands to Crewe, is expected in July.
A consultation on the current route refinement for Phase 2b (from Birmingham to Leeds and from Crewe to Manchester) ended last month, with a route to be finalised by the end of this year, followed by a further EIA.
The Woodland Trust says its campaiging has already had "some significant wins" on the Phase 1 from London to Birmingham which was granted Royal Assent in February, including forcing HS2 Ltd to remove all temporary works from ancient woodland, and helping to make the case for an extended tunnel through the Chilterns, so saving 9 hectares of ancient woodland from three separate woods.
Speight added: "These wins should now set a precedent for how the second phase of the project is handled. We expect HS2 Ltd to take more seriously the need to avoid ancient woodland and for it to make plans and amendments to the route that reflect this need as the process continues."