When Taylor Swift returns to the stage, London authorities and Wembley Stadium will be taking extraordinary steps for security — including some specifically deployed to stop potential terrorists … TMZ has learned.

Law enforcement sources tell TMZ … Taylor’s 3 upcoming shows at Wembley will be relying on additional private security specializing in counterterrorism, partly in response to the Vienna terror plot foiled by U.S. and Austrian officials.

We’re told the private security force will also be deployed because of ongoing London riots that are placing a major strain on local police.

Our sources say Wembley, in particular, commonly has high-level security measures in place, especially compared to similar venues, and it’s one of the safest in Europe from a counterterrorism perspective … but there are still concerns.

Our sources say the big issue for Taylor’s upcoming Wembley shows will be protecting the throngs of Swifties that always gather outside her concert venues.

We’ve already seen huge crowds outside her other European concert stops, like the ones in Munich for example.

Fact is, her rabidly loyal fans congregate outside venues, even if they don’t have tickets, and they’ll need protection, too.