Worthington, who has been in charge since Lawrie Sanchez walked away in 2007 to take over Fulham, has been under increasing pressure in recent weeks after a home defeat to Serbia was followed by back-to-back losses against Estonia.
That wrecked any chance the side had of securing a Euro 2012 play-off and they face the Azzurri lying fifth in Group C, ahead of only the Faroe Islands.
Some supporters produced banners and called for Worthington's head after Friday's 2-1 defeat to the Estonians, while Sanchez made some scathing remarks about his successor in a recent radio interview.
A handful of senior players, including Chris Baird, Chris Brunt, Maik Taylor and David Healy, rallied around the boss, insisting he should be retained by the Irish Football Association when his existing deal expired on December 31, but Worthington conceded his time had come.
Addressing a press conference only minutes after informing the squad of his decision, Worthington said: "The situation is that as from tomorrow night [Tuesday] I will step down as manager of the national team.
"Up until three games to go we were in a very good position but, through no one's fault, sometimes the game of football throws you a raw hand.
"I feel the time has come after four and a half years, which have been fantastic for me as an individual.
"Sometimes when you try to get blood out of a stone there's only so much you can get. I think I've gone as far as I can with the group we've got.
"There were genuine mistakes in the last three games, something that was not there in the previous matches.
"I probably half-thought about (stepping down) after Estonia away, but that wasn't something I would show to the press because I wanted to fulfil my promise to the players, to the board and to the supporters."
Worthington refused to criticise the fans for their recent barracking of him, but called on them to put on a united front whenever a new man is appointed.
"The decision was not through supporter pressure because I can deal with that. It's a football decision that I think is right," he said.
"The question was asked of me at the outset 'would I turn my back on my country?'. I said no then and I was determined to see it through and that is what I've done.
"All I would say to the fans is to be behind the team.
"They are phenomenal supporters and the way they travel around Europe in huge numbers is second to none. From a players' point of view and from the manager's point of view that is much valued.
"Of course frustration and disappointment creeps into the game, I fully understand that. But the fans have been wonderful with me for many years as a player and up until recent they have been wonderful again as manager."
Despite those words, Worthington sounded one worrying note of caution for the future.
He suggested that recent mood surrounding the side - from the stands and from the local media - could end up driving away important players.
"The one thing I think supporters have to be careful of is that negativity around the football, and from some of the press," he said.
"If they're not careful they will switch some of our players off and they will pack it in.
"Representing your country is a fantastic honour but some people deal with situations better than others. Some might think 'I don't want to be around this mood, this feeling'.
"It's something that has been murmured and you wouldn't want to see players stepping away from honouring their country through those circumstances.
"When Windsor Park is bouncing it's a fantastic place to be but please remember, as the song goes, we're not Brazil, we're Northern Ireland. Sometimes that can be easily lost."
Assistant manager Glynn Snodin and goalkeeping coach Fred Barber will also finish after Tuesday's match, while under-21 boss Steve Beaglehole's position will be reviewed at the end of the year.