Company executives only allow guests aged 18 and over, or kids travelling with an adult, access to the exclusive airport rooms, and as a result, the 17-year-old actress was refused entry as she waited for a solo flight on Saturday night.
Ranting about the ban on Twitter.com, she wrote: "Denied from the business lounge once again for being under 18 and without an adult.. @British_Airways sort your s**t out.
"I'm not saying I deserve special treatment... Just saying if 16-18 yr (year) olds are grown up enough to travel 5,400 miles around the world... I'm sure we can handle sitting quietly in the damn lounge."
Her rant was picked up by Ed Sheeran, who tweeted: "Ah British Airways. You failed again."
A representative for the airline attempted to appease the actress by insisting rules are rules, stating: "Hi Maisie. We're sorry if you were disappointed, lounge guests must be 18 or older. We look forward to welcoming you soon."
Williams turns 18 in April.