The pair were rumoured to have become engaged in December last year when sources also spoke about Michelle's pregnancy.
Williams and Kail worked together on the biographical miniseries Fosse/Verdon and were spotted together in London at the end of last year where Michelle had been shooting scenes for Venom 2.
The couple was also seen buying baby clothes at Seraphine's maternity boutique in Kensington, in preparation for the arrival of their baby.
Michelle has spoken candidly about her search for love following Ledger's death in 2008.
The Hollywood star admitted she always remained optimistic that she would one day find someone who could fill the void in her life.
Speaking in 2018, she said: "I never gave up on love.
"I always say to Matilda, 'Your dad loved me before anybody thought I was talented, or pretty, or had nice clothes.'"
Michelle has made a concerted effort to keep her personal life away from the spotlight. But she also recognises that talking about her own issues could help other people deal with their problems.
She said: "I don't really want to talk about any of it. But what if this helps somebody?
"What if somebody who has always journeyed in this way, who has struggled as much as I struggled, and looked as much as I looked, finds something that helps them?
"Don't settle. Don't settle for something that feels like a prison or is hard, or hurts you. If it doesn't feel like love, it's not love."
