The sandwich - called The Imposter - launched on June 17 in selected restaurants in London, Bristol, and the Midlands*, for a four-week trial.
It features a bespoke Quorn fillet, coated in the chain's Original Recipe herbs and spices, lettuce, and vegan mayo in a soft-glazed bun. According to the fast-food giant, it is aimed at vegans and meat-eaters alike.
According to KFC, The Imposter has racked up staggering sales figures, around 500 percent higher than an average new KFC sandwich.
"We knew there were plenty of people who would want to get a taste of the Imposter, but we didn’t anticipate this phenomenal response," Victoria Robertson, Senior Innovation Leader at KFC, said.
"We can't wait to get the Imposter back out in restaurants so vegans can get their finger lickin' fix."
Although the burger has sold out across the UK, not all vegans were keen to get their hands on one, with many citing animal cruelty as a reason to boycott the chain.
"KFC murder millions of chickens every year. Just because they now have a vegan option, it doesn't make the chain suddenly cruelty-free. If you buy the vegan burger, you are still paying to hurt chickens," one social media user said.
"You're still putting money into the coffers of company that makes its primary source of money off of the slaughter of a specific animal...this is just a marketing ploy," another added.
The Imposter will be back on the menu from July 2.
?*Stores taking part in the trial include:
- Bristol - Fishponds
- Bristol Bradley Stoke
- Bristol - Winterstoke Road
- Bristol - Eastgate Retail Park
- Bristol - Avonmeads Retail Park
- Bristol - Shield Retail Park
- Bristol - Keynsham
- Bristol - Hengrove Leisure Park
- Bristol - The Venue Cribbs Causeway
- Birmingham - Great Bridge Retail Pk
- Bloxwich - Leamore Lane
- Cannock - Orbital Retail Park
- Tamworth - Ventura Retail Park
- Tyburn - Kingsbury Road
- Walsall - Junction 10 M6 Services
- Walsall - Park Street
- Dudley - Merry Hill Food Court
- Stourbridge - St Johns Road
- Dudley - Merry Hill Drive Thru
- London – Gloucester Road