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Today's the day when we should learn who this "surprise witness" is (being represented by celeb-lawyer Gloria Allred) in the custody battle between Britney and Kevin. X17 will be there, mic in hand, to get comments from Allred about the witness reported (by Fox News) to have possible eyewitness testimony of Britney doing drugs in front of her kids. The Fox report says Britney's kids could be taken away from her as early as today!

For her part, Britney's side says this it a huge pile of BS -- they maintain that Britney has never done anything in front of the kids that she would be worried about and that therefore, she is unconcerned about whoever this surprise witness may be.

Our source tells us it's assumed it will be one of three bodyguards fired by Britney shortly after her stint in rehab. He suggests a disgruntled former employee may be seeking revenge but that there is no truth to whatever the witness might say of Britney being an unfit parent -- he suggests if an employee had seen something troubling, then he/she would have reported the activity to officials or law enforcement at the time.

In an exclusive interview with X17 Sunday evening, Allred said, “We believe our client is a very important witness.”

When asked if his testimony might be strong enough to have the kids taken from Britney, she replied, "I don’t want to comment on what his testimony will be, except to say that the we believe the judge should have all the relavant testimony."

When asked how she came to represent the mystery witness, she answered, “He came to me. I’ve been practicing for 31 years and people know what I do. I can’t tell you his exact motive, but I would characterize him as a reluctant witness."

Click HERE for Allred's statement about the whole thing ...

Stay tuned for our coverage of this tomorrow! ...