Make-up artist Lisa Armstrong, 41, tweeted an image of two broken hearts and a whole heart just after 5am on Sunday.
Just hours before, it was revealed McPartlin – one half of presenting duo Ant and Dec – is believed to be seeing Anne-Marie Corbett, his 42-year-old personal assistant.
On Sunday afternoon, in reply to English glamour model Nicola McLean’s concern for her, Armstrong said “she was MY friend!!”.
In reply to a fan, she also said “my friend and OUR PA who I let into OUR home”.
His wife, who is still going through divorce proceedings with him, also revealed she only found out they were dating through the media, and not from McPartlin.
The TV presenter, 42, has reportedly been supported by Corbett over the past few months since he crashed his car while drink driving in Mortlake, southwest London in March.
He was fined £86,000 and given a 20-month driving ban, prompting him to get help in rehab.
Corbett, who has two young children, has been described as “his rock”, showing up at the police station when he was arrested and in court when he received his ban in April.
“He’s had a tough time of it, but Anne-Marie has put a smile back on his face again. She has been his rock,” a source told The Sun on Sunday.
“They have been living in each other’s pockets, day in, day out.
“Knowing someone is there for him has brought him back from the brink – she’s put him back together again.
“It’s very early days, but everyone is hoping he can put the worst year of his life behind him and find happiness again. Both of them deserve it.”
McPartlin and his wife, who do not have any children, were together for 24 years and married for 11 years before filing for divorce in January.
Their relationship is said to have suffered after he became addicted to painkillers following knee surgery.
They both work on Britain’s Got Talent and it is assumed they will continue to, although Armstrong is no longer working on Saturday Night Takeaway, presented by her estranged husband and co-presenter Declan Donnelly.
Corbett separated from her husband Scott in October last year, with sources stressing McPartlin was nothing to do with her marriage break-up.