A council is to seek a civil injunction and damages against one of its own councillors and other members of the public.
Nether Edge Green Party councillor, Alison Teal, was served with a legal notice for her involvement in protests against Sheffield City Council's tree felling programme.
Ms Teal was previously arrested for protesting against the tree removals.
Charges against her and 13 others were dropped earlier this year.
Sheffield City Council maintain that the removal of street trees in the city is lawful and necessary.
A "pre-action protocol letter" from the council's legal department to Ms Teal warned the authority will seek a High Court injunction to stop her, and others involved in "direct action", from taking part in future protests.
The council said the process would give protesters "a chance to further and finally consider their position" before it begins legal proceedings on 12 July.
Councillor Bryan Lodge, cabinet member for Environment and Street Scene, said there had been months of "unlawful and costly disruption" to tree replacement works in Sheffield by a small number of protesters.
"We continue to support the right to peacefully protest, and the majority of protesters who are doing so peacefully will not be affected," he added.
"But there is a big difference between this and direct action which deliberately and unlawfully stops works from being carried out."
Mr Lodge said protesters had been sent a letter several weeks ago confirming their actions were unlawful.
"Streets Ahead is not just about street trees, it's about ensuring we have roads, pavements and a street scene to be proud of for many years to come," he added.
"In addition, it presents us with a unique opportunity to sustainably manage, increase and maintain our diverse street tree stock over a 25 year period."
The council said, despite the letter, works were still being disrupted "causing city-wide delays".