A train has struck a tree near Royston, with no trains running between Letchworth and Cambridge as a result.
The incident occurred when the 1547 Ely to London King’s Cross service struck a tree, which has blocked both lines.
As a result services travelling from London will terminate and restart from Letchworth, with replacement buses due to be in place around now (6.15pm).
In a tweet on its @GNRail account, rail operator Govia Thameslink said: “Our current advice is to avoid all but essential travel. Services are being subject to severe delays and are likely to be cancelled.
“Replacement buses are extremely limited and will be very busy.”
Emergency responders from Network Rail, the British Transport Police and the fire service have all been called to the scene.
On its website, rail operator Govia Thameslink has posted: “Large obstructions such as trees can cause issues for a considerable amount of time.”
It currently estimates the disruption to continue until 9pm.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here