The pair split in December

The 39-year-old split from the Stranger Things actor in December after the pair, who had been married for five years, had been living separately for months.

In January, the Smile singer was reportedly 'in agony' after discovering the 49-year-old had an active profile on celebrity dating app Raya.

Now, according to MailOnline, the London songstress has been dealt a fresh blow as allegations emerge that her spouse cheated with a costume designer during their relationship.

A source told the publication: "David cheated on Lily. They met on a movie they did together. They weren't very secretive about their relationship.

"David flew her to Atlanta while he shot Stranger Things to keep him company."

The actor purchased a house in Atlanta last year to be closer to the Stranger Things set, while Allen is planning to return to London with her two daughters who she shares with ex-husband Sam Cooper.

Following the marriage breakdown, Allen checked into a £10,000-a-week American clinic, reportedly in 'emotional turmoil' and 'highly medicated' on antidepressants.

On her Miss Me podcast, Allen admitted: "I'm finding it really hard to be interested in anything. I'm really not in a good place. I've been spiralling and spiralling and it's got out of control.

"I can't concentrate on anything except the pain that I'm going through."

Meanwhile, Harbour is now reportedly dating 27-year-old model Ellie Fallon who is 22 years his junior.