The hoodies began in Newquay, but have spread around the South West
Hundreds of people are supporting a move to ban the sale of replica lifeguard sweatshirts.
The red hoodies, emblazoned with Lifeguard, have exploded in popularity around South West seaside resorts.
But campaigners, as well as the RNLI, fear tourists might mistake those wearing them for real lifeguards.
Matthew Perry, of Hayle Surf Lifesaving Club in Cornwall, has launched a campaign on a social networking site which has nearly 2,000 supporters.
The red and yellow colours are recognised around the world
Mr Perry, who is organising a petition to give to Restormel Borough Council asking it to request shops to stop selling the garments, said: "I have seen adults walking along the beach together wearing these hoodies.
"Sometimes they even try to get their picture taken with the rescue equipment.
"But impersonating someone who is trained to save lives, costs lives."
The hoodies and T-shirts in the red and yellow colours of the RNLI lifeguards have been selling in Cornwall for the last five years.
But this year their popularity spread and they have been seen in Devon resorts.
Steve Instance, RNLI lifeguards inspector for the South West, said the fake goods presented a "massive" problem because the red and yellow lifeguard colours were recognised globally.
He said he had no problem with the garments in other colours.
He said: "There's no way we can change our colours, the red and yellow pattern is recognised in nearly 100 countries around the world by the International Lifesaving Federation."
While there was no legal ban on impersonating a lifeguard, wearing the fake goods was "stupid".
He said: "If you were in a panic you could be easily misled by these garments.
"Lifeguards are highly trained and skilled people and these clothes have the potential to risk lives and we support any move to get them banned."
He added: "I can understand why people like the image - it's a healthy lifestyle and people look up to lifeguards - but they should not be worn on or anywhere near the beaches."