Robert De Niro has split from his wife Grace Hightower after 21 years of marriage.

The 75-year-old Casino star and his 63-year-old wife Grace 'are no longer living together' according reports in Page Six.

The long-term couple are expected to 'announce their breakup publicly in the coming weeks,' alleged the publication.

Happier times: Robert De Niro and his wife of more than 20 years, Grace Hightower, have split. The couple seen here in April 2018

The Raging Bull star, who shares 20-year-old Elliot and six-year-old Helen with his wife, was spotted on the red carpet on Monday at a MoMa event in New York City without the former flight attendant by his side.

It is not the first split for the couple, who were wed in 1997, before Robert filed for divorce two years into their union however the arrangement was never finalised.

Robert and Grace eventually reunited, renewing their marriage vows in 2004.

The actor is notoriously private about his relationship with Grace but the pair are outspoken about their belief that their son Elliot's autism is linked to vaccinations.

All by myself: De Niro was seen solo at a New York City event on Monday (pictured)

Explaining how Grace believes the his son 'changed overnight' after being vaccinated as a child, Robert told Today in 2017: 'The vaccines are dangerous to certain people who are more susceptible...

'Nobody seems to want to address that, or they say they've addressed it and it's a closed issue but it doesn't seem to be.'

'There are many people who will come out and say I saw my kid change overnight. I saw what happened, I should have done something and I didn't. There's more to this than meets the eye, believe me.'

Family matters: [L-R] Scott J. Campbell, Jarrod Spector, Jessica Keenan Wynn, Robert De Niro, daughter Helen Grace De Niro, Chilina Kennedy, Liz Larsen, Paul Anthony Stewart and Grace Hightower in 2015

Daughter: Drena De Niro seen here in 2016. She was adopted by Robert while he was married to her mother, Diahnne Abbott

The Meet The Parents actor had previously been married to actress Diahnne Abbott, from 1976 to 1988.

The pair had a son, Raphael, in 1976 while the Oscar-winning actor adopted Abbott's daughter Drena from a previous relationship.

The Mean Streets star also has two children with ex Toukie Smith, twin boys Aaron and Julian.

Early days: Robert and Grace seen here at the premiere of Ronin in 1998

The twins: Robert's twin boys Aaron and Julian are seen here in 1998

De Niro spoke about his feelings on marriage in a 2013 interview with Katie Couric.

The star of The Godfather Part II said that the secret to a happy union involved 'a lot of give and take, standing your ground, because then you keep the respect you started out with.'

However, he shared some rather more cautious advice with The Independent in 2014, saying: 'The only thing I would say is that if you get married, it's easier probably not to have children if you're going to split up – it makes life simpler.'