Taylor Swift Wembley Concerts Using Counterterrorism Security Forces

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 5 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.

Ben Knott, operations director at FGH Security in England, tells TMZ ... private security firms work with police outside of events where fans gather, and they're usually tasked with sniffing out suspicious people.

He says counterterrorism security forces tend to deploy plainclothes and uniformed personnel, whose job it is to look for strange people in the crowd. If someone's scanning the area for cameras and police, that would be a sign something's not right, and security would approach.

In most cases, we're told the tactic is to be friendly with the suspicious individual, and ask if they need help with anything -- but if security gets the slightest whiff of anything nefarious, police will be called in immediately.

The heightened security, and the emphasis on counterterrorism measures come in the wake of Taylor's canceled concerts in Vienna ... where 3 suspects have been arrested for allegedly plotting to detonate explosives inside Taylor's show, and inflict mass casualties on fans outside the venue.

Officially, London Metropolitan Police says, "There is nothing to indicate that the matters being investigated by the Austrian authorities will have an impact on upcoming events here in London."