Humes will marry The Saturdays singer Rochelle Wiseman in July, and his fellow bandmates have hired an ex-MI5 operative to help them with their special plans.
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Oritsé Williams told The Sun: "I have met up with an ex-MI5 guy who is helping us plan Marvin's stag do.
"We are taking this thing deadly seriously and will be rehearsing it. It is a covert MI5 operation.
"This guy has told us he will help us get it done in the most discreet way so Marvin will have no idea what will hit him."
Williams explained that he wanted to get Humes back for pranking him on his birthday by hiring a stripper during their G-A-Y performance in London.
"After what Marvin did to me on my birthday I am going to make sure his life is hell," he added. "It was the most shocking thing that has ever happened to me. I'm not letting it go."
Humes recently revealed that he "stuttered and stumbled" while proposing to Wiseman.
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