'I felt like the worst mum in the world': William Tyrrell's mother breaks her silence

The mother of William Tyrrell has spoken for the first time about her missing son, who disappeared without a trace three and a half years ago.

Karlie Tyrrell has opened up in an exclusive interview due to air on Channel Seven's Sunday Night, where she reveals her anguish after three-year-old William vanished while playing at his foster grandmother's front yard in September 2014 while wearing a Spiderman costume.

"I felt like I was the worst mum in the world," she tells Sunday Night's Mel Doyle.

It is widely believed William was abducted while playing with his sister at the home in Kendall, NSW, prompting Ms Tyrrell to offer strong words to whoever may have been responsible for his disappearance.

"Whoever has him needs a bullet," she says.

Despite extensive police resources being channelled into William's disappearance, one of Australia's biggest manhunts failed to find the missing boy or anyone responsible.

As his disappearance approaches its fourth anniversary, Ms Tyrrell will reveal to Sunday Night who she believes should be held accountable.

"They were responsible, they failed," she says in the upcoming interview.

• New revelations about the lead up to NSW toddler William Tyrrell's disappearance

• Revealed: Missing William Tyrrell was in foster care at time of disappearance

New, unreleased facts regarding the case are also due to be revealed.

Sunday Night returns at 8.45pm this Sunday March 4 on Channel Seven.