The huge white message on grass can be seen from planes as they arrive at Gatwick and was photographed by Abbey Desmond from Essex on Saturday.
She said she flew into Gatwick at 8.45am from Cancun.
Abbey posted the image on Twitter and said: “Flying into Gatwick, just before landing this is what is out the left window… great prank, deffo at Gatwick.”
Other passengers posting on Twitter said they had been confused, worried and delighted by the sign.
Popular Youtuber Max Fosh later revealed that he was behind the prank on the Justin Dealey show (BBC Three Counties Radio).
Max, 27, who currently has more than 924,000 YouTube subscribers, also retweeted Abbey’s tweet on Saturday evening with the message ‘Guilty…’.
Max said the idea came from similar pranks that had been pulled in America and Australia.
He said: “There is a guy who has done it for the past 30 years in Cleveland and I wanted the UK to have its own version.”
Max explained that the tarpaulin for the sign was assembled and put together by an artist called Angus.
He said: “He made each letter individually and we then went to the field to install it.
“It was someone’s personal land and they have given me access to use it for the next six weeks so it’s going to be up there for a while.”
Max said he is ‘really happy’ with the way the project went: “I’ve had a few passengers get in touch to say that there was mild worry for about six seconds before realising that it can’t be right.
"But everyone has seen it as some silly fun.”
He added: “With all my projects I try to make sure that they are just silly and don’t inconvenience anyone.
"And I think this is exactly what’s happened on this occasion.”
Gatwick Airport has been approached for comment.