Updated

Don’t mess with the royal wedding.

That’s the message British authorities are sending about security arrangements as Prince Harry and Meghan Markle plan to tie the knot Saturday.

Snipers in full military gear roamed London’s streets — with officers given orders to “do whatever is necessary,” including shooting to kill, The Sun reported.

“Operations will have been planned for months,” security expert Will Geddes told the Daily Star. “Police will have mapped the area, established risk profiles and worked out the modus operandi of high-risk groups.”

He added: “Police will be looking for a variety of threats,” including ISIS terror threats, stalkers and YouTube pranksters.

The $40 million security bill for a “ring of steel” perimeter is even higher than that for Prince William and Kate Middleton’s wedding day, according to The Sun.

The country has been on high alert after a spate of terror attacks, including incidents on London Bridge, Westminster Bridge and Manchester Arena.

Vehicular attacks, as well as those by a lone attacker with a knife, could spark pandemonium in the crowds. The objective of such attacks would be publicity, Geddes said

“The world will be watching — nothing can afford to go wrong,” he said.

Police also have asked the public not to toss confetti or other festive items during the event — citing security and sanitation concerns, according to the Windsor & Eton Express.

“Please do not throw confetti or any other items at any time during the day, particularly during the procession. It poses a potential security risk and it’s a bit of a pain to clean up!” the Thames Valley Police tweeted.

This article originally appeared on Page Six.