(Adds Enbridge share price, background on Line 5 dispute)

By Nia Williams

CALGARY, Alberta, Oct 20 (Reuters) - Enbridge Inc shut down its Line 5 oil pipeline for several hours on Tuesday after protesters trespassed onto a facility in Michigan and tampered with the pipeline, an Enbridge spokesman said on Wednesday.

Line 5 is at the centre of a long-running environmental dispute between Calgary-based Enbridge and the state of Michigan that has embroiled the Canadian and U.S. governments.

Michigan Governor Gretchen Whitmer ordered the company to stop operating the pipeline by May because of concerns a section running underwater in the Straits of Mackinac could leak into the Great Lakes. Enbridge ignored that order and the two sides are embroiled in a legal battle over Line 5's fate.

Earlier this month the Canadian government, which backs Enbridge, escalated the dispute by invoking a decades-old pipeline treaty with the United States to trigger negotiations with Washington.

Line 5 ships around 540,000 barrels per day of crude and refined products from Superior, Wisconsin, to Sarnia, Ontario. Enbridge spokesman Ryan Duffy said the line was back up and running and the company did not anticipate any impact on deliveries to customers.

Environmental campaigners said a protester had turned a safety valve on the pipeline.

"The water protector called Enbridge so that they could safely halt the flow of oil before reading a statement and turning the valve to halt the operation of the pipeline," said an emailed statement from the Resist Line 3 Media Collective, which is also opposed to Enbridge's Line 3 pipeline.

Whitmer has previously said Line 5 poses an "unacceptable risk" to the people of Michigan, and that an oil spill in the Great Lakes would be catastrophic.

Line 5 is a key part of Enbridge Mainline network, which delivers around 3 million barrels per day of western Canadian crude to refineries in the United States and eastern Canada.

Duffy said Enbridge would seek the prosecution of all those involved in Tuesday's protest.

"The actions taken to unlawfully trespass on our facility in Michigan and attempt to tamper with energy infrastructure was reckless and dangerous," Duffy said.

Enbridge shares were flat at C$53.09 on the Toronto Stock Exchange. (Reporting by Nia Williams; Editing by Andrea Ricci, Bernadette Baum and Bill Berkrot)