Race organisers Singapore GP said yesterday that they are investigating an alleged breach in track security at the Marina Bay Street Circuit.
This comes after a post on website All Singapore Stuff surfaced yesterday morning that reportedly showed a couple of fans - one of whom was wearing a McLaren Honda team shirt - posing on the race track.
In the Facebook post, the caption read: "3am in the morning. Bribed the security guard to let us onto the track. He not only agreed, he took the photo. Non F1 fans won't understand this, but most of us will".
A Singapore GP spokesman told reporter last night: "The safety of our patrons is of utmost priority and we view the allegation of bribery seriously.
"We are working with SRS, the appointed security agency assigned to the reported area, to investigate the claim."
In 2015, Briton Yogvitam Pravin Dhokia went onto the track midway through the Singapore Airlines Singapore Grand Prix race via a gap in the fence, and walked across the Esplanade straight.
The then-27-year-old was sentenced to six weeks' jail for committing a rash act that "endangered personal safety of the drivers".
Then, in last year's race, a race marshal who was removing debris had to scamper across the main straight as the cars charged towards Turn 1 after a safety car restart. Mercedes' Nico Rosberg, who was leading the field, had described the incident as "pretty hairy".
A day earlier, a giant monitor lizard had entered the track during Saturday's practice. Red Bull's Max Verstappen could be heard telling his team on the radio: "There's a giant lizard on the track."
This year is the 10th edition of the Singapore race, which has been extended until 2021.