The China In Your Hand star, who is currently on tour with fellow 1980s acts Nik Kershaw and Go West, bought a product through Fabletics' online store – fronted by Hollywood actress Kate Hudson – in April. But last week she discovered that since then the firm had allegedly taken £300 from her PayPal account.
Decker, who was educated in Wellington, Telford, and launched her music career performing in the county, said: "I made one purchase but didn't realise in the process I'd been scooped up into their monthly membership scheme.
"I only noticed they had taken £44 a month from my account when I did a bit of admin. All their emails had gone into spam, so I didn't even know about the subscription or that I could stop it.
"With spam filters, it must happen to lots of people. The company seems to be totally reliant on people being too busy or unaware about the charges to stop them.
"In my view they're guilty of unethical sharp practices."
The firm said: "Fabletics provides at least 15 separate notifications – including six before a customer checks out for the first time – about the subscription model."