Sexton, the Commonwealth champion, suffered a cut hand in training forcing him out of next month’s bill.
Unbeaten Chisora had been due to meet Williams last year but was banned after biting opponent Paul Butlin on 22 May.
"I wasn’t expecting to get this chance, but I’ll take it with both hands," said the 26-year-old from Finchley, London.
Chisora was banned for four months and fined £2,500 for the incident which took place at York Hall in Bethnal Green but returned in October to stop Georgia fighter Zurab Noniashvili in the eighth round.
The Zimbabwe-born fighter is rated as one of the brightest prospects in the British heavyweight division.
"It was my own fault the fight with Williams didn’t go ahead, and I’ve been kicking myself ever since," added Chisora.
"This is a massive opportunity to really make a statement and become the top man in Britain."
Chisora is unbeaten in 11 fights, winning six inside the scheduled distance, and had been due to meet Matt Skelton for the Southern Area title on the undercard of Williams-Sexton.
Williams has eight defeats from 50 fights and intends to retire after the meeting with Chisora having admitted recently he was past his best.
The 36-year-old from Brixton, London is best known for his stunning fourth-round victory over former heavyweight king Mike Tyson in 2004.
But he looks set to call time on his 15-year professional career after February’s bill which also sees British stars Kevin Mitchell, Kell Brook, Nathan Cleverly, James De Gale and Frankie Gavin in action.
"Chisora has got a reputation as a biter, but then so did Mike Tyson and look what happened to him," said Williams.
"I don’t care whether I fight Sam Sexton, Derek Chisora or King Kong, I’m going to war one last time and I’ll put it all on the line."