IT’s the Battle of the B’s – with Britney Spears blaming Beyoncé for her poor album sales!

The troubled “Toxic” singer’s heavily hyped eighth album, “Britney Jean,” debuted in a disappointing fourth place on the “Billboard” charts, making it the lowest-selling release in her career.

But Beyonce’s self-titled surprise album – which dropped with no pre-release promotion – shattered records to become the fastest selling ever on iTunes.

Now sources say Britney is lashing out at Beyonce!

 “Britney’s album is a flop, but rather than take responsibility, she’s say­ing it’s all Beyonce’s fault,” an insider told The ENQUIRER.

“As crazy as it sounds, Britney honestly believes Beyonce deliberately timed the release of her album to humiliate her.” Even worse, when Britney realized Beyonce’s album was trouncing hers, the “I’m a Slave 4 U” singer went on a rampage, said the source.

Britney knocked a computer to the ground and forbade anyone in her camp from mentioning Be­yonce’s name or playing her music,” revealed the insider. “Britney also went off on her dad Jamie, screaming at him and accusing him of mismanaging her career.”

With her reign as pop princess in jeopardy, friends fear the 32-year-old could be headed for another emotional collapse.

Britney suffered a complete psychotic breakdown in 2008 and was carted off to a mental ward at Cedars-Sinai Medical Center.

She remains under a court-ordered conservatorship, and sources say Britney’s latest meltdown threatens to derail her upcoming marriage to beau David Lucado.

As The ENQUIRER reported, the twice-divorced singer accepted the 28-year-old Virginia native’s proposal recently and began planning a wedding in Hawaii.

“Britney thought she’d wrap up 2013 as the queen bee of pop,” said the insider. “But Beyonce has clearly stolen that crown.”