A source claimed that the couple – who are yet to confirm they are dating despite speculation – are growing closer by the day.
It comes after it was reported earlier this month that Naomi and Liam were ‘all over each other’ backstage at the Formula E Hong Kong E-Prix, despite arriving separately and refusing to be photographed together.
The source said: ‘This is a big step for them. It is like she is coming out with the romance.
‘A lot of her friends thought the relationship might fizzle out – but the opposite seems to be happening.
‘Naomi has kept him hidden away before but he is at the centre of her birthday plans.’
MailOnline has contacted representatives for Naomi Campbell and Liam Payne for further comment.
Naomi and Liam were first rumoured to be dating in January, and despite being seen arriving at the same events, they have never been photographed together.
Earlier this month, the pair were reportedly getting very cosy backstage at the Formula E Hong Kong E-Prix, but refused to be photographed together.
A source told The Sun: ‘Naomi and Liam were acting like love struck teenagers on their first foreign holiday when they were out of the public’s gaze.
‘She is very tactile and Liam made sure he poured compliments on her as they spent time together at the event.
Earlier this week, the two stars also did little to quash romance rumours as they made separate appearances at WE Day.
During her appearance, Naomi cut an interview short and managed to dodge a question regarding Liam when quizzed on the red carpet, according to The Sun.
An onlooker told the publication: ‘Naomi was having the time of her life. At the party she was getting quite close to Drake – they ended up dancing together.
‘She doesn’t want to be in a serious relationship and hasn’t been inviting Liam to events. She just doesn’t seem that fussed and likes her space.’
Rumours of the pair’s fledging relationship have been circulating the showbusiness circuit since Liam flirted on a snap posted to Naomi’s Instagram account, commenting: ‘Perfection in a person…don’t give me those eyes.’
Naomi then wrote alongside a shirtless picture of Liam, ‘Beautiful soul,’ to which he replied: ‘Takes one to know one,’ and added a kiss emoji.