The start of the trial in Waymo's suit against Uber over swiped self-driving car secrets was delayed once again Tuesday in the face of potentially troubling new evidence.

Waymo and Uber confirmed that US District Judge William Alsup postponed the trial, which was scheduled to begin next week, to a date yet to be determined.

Last-minute evidence reportedly came in the form of a letter indicating that Uber may have had a system in place to gather information regarding rivals or regulators and store it on shadow servers to keep it at arm's length.

The letter was uncovered as part of a separate criminal investigation by the US Attorney's office, and suggests Uber used secret servers for storing data it didn't want traced to the company, according to tech news website Gizmodo.

The letter, said to be by a former Uber global intelligence team member, was given to the court just days ago by US attorneys.

"The evidence brought to light over the weekend by the US Attorney’s office and revealed, in part, today in court is significant and troubling," a Waymo spokesperson said in response to an AFP inquiry.

The case stems from a lawsuit filed in February by Waymo -- previously known as the Google self-driving car unit -- which claimed former manager Anthony Levandowski took a trove of technical data with him when he left to launch a competing venture that went on to become Otto and was later acquired by Uber.

Uber fired Levandowski earlier this year, just ahead of a date set by a judge for Uber to return files taken from Waymo.

Uber has held firm that it never got any of the data Waymo says was taken.

Alsup in October granted a request by Waymo to delay start of the trial so attorneys could dig into an internal Uber report turned over late in the evidentiary process.

"Uber has been waiting for its day in court for quite some time now," an Uber spokesperson told AFP.

"None of the testimony today changes the merits of the case."

Waymo's lawsuit contends that Levandowski in December 2015 downloaded files from a highly confidential design server to a laptop and took the data with him to the startup.

Waymo argued in the lawsuit that a "calculated theft" of its technology netted Otto a buyout of more than $500 million and enabled Uber to revive a stalled self-driving car program.

Uber acquired commercial transport-focused Otto late last year as the company pressed ahead with its pursuit of self-driving technology.

Levandowski, a co-founder of Otto, headed Uber's efforts to develop self-driving technology for personal driving, delivery and trucking