“Four years is an eternity as Barca coach,” the 41-year-old Guardiola, dressed in a grey V-necked sweater and white collared shirt, told a news conference packed with journalists and players.
“That’s four years, with time everything runs out of steam. I am drained and I need to take a step back and replenish.”
Guardiola stood and embraced club president Sandro Rosell after making his statement from the podium.
“Boys, I’m leaving Barca,” Guardiola told his players according to an online report by the biggest selling sports daily Marca, which did not identify a source.
The coach will announce his decision at 1.30pm (1130 GMT) at the Camp Nou along with president Sandro Rosell and football sporting director Andoni Zubizarreta, the club said online.
After spending most of his playing career at Barcelona, Guardiola coached Barcelona’s B team before taking control of the first-team squad in June 2008 from Dutchman Frank Rijkaard.
In his four years at the helm, he has led Barcelona to 13 titles and is credited with promoting some of most sublime football in the world, helped by huge talent including footballer of the year Lionel Messi.
“Pep is going” cried the front-page headline of Barcelona-based daily Sport, adding, however, that while his decision probably was already taken there was still a glimmer of hope.
“Where will you be better off than in Barca?” the paper pleaded beneath a photograph of the 41-year-old coach, head down and hands pressed together in deep reflection.
“Turn the front page around depending on what Guardiola says today,” the paper advised its readers.
Inside, however, the paper was more pessimistic. “He has practically decided to leave Barca: today he will tell the management and the team,” Mundo Deportivo said.
“Nevertheless, up to this morning the door was still open for him to carry on.”
No-one at the club was available to comment.
Chelsea knocked Barcelona out of the Champions League semi-finals this week and the Catalans’ archrivals Real Madrid beat them in a pivotal league clash at the Camp Nou last weekend.
Real Madrid, leading Barcelona by seven points at the top of the league, are now all but certain to be crowned Liga champions.
Earlier this week the daily Sport said candidates to replace Guardiola included France coach Laurent Blanc, Olympiakos coach Ernesto Valverde and Athletic Bilbao’s Argentine coach Marcelo Bielsa.
In April last year Guardiola told Italian television Rai Sports that he felt his time as Barcelona coach was coming to a close.
“I feel good here but I believe my time at Barcelona is ending,” the coach said at the time.
“I’ve already done three years, one more will be four years. And in four years a big club needs a lot of courage to keep the same coach. Everyone should know when it’s the right time to go, as I always heard when I was a player.”
The coach extended his contract with Barcelona by a season in February 2011 and his deal expires in June this year.