Long had been one of the Lions' star performersGreat Britain's half-back Sean Long has flown home from the tour of Australia and New Zealand - with the reasons for his departure still unclear.
A team statement said Long, whose partner is heavily pregnant, had returned home "for personal reasons".
But the BBC's Dave Woods says there are allegations that Long broke a team alcohol ban on Sunday.
Britain need to avoid defeat against Australia in Brisbane this Saturday to make the Tri-Nations final.
Early reports from Australia claimed that St Helens half-back Long had been accused of gambling on matches in the Tri-Nations series.
But Woods said he was "99%" sure that those allegations were incorrect.
Woods said that Long had been seen inebriated on Sunday, the day after the Lions had been thrashed 34-4 by New Zealand in Wellington.
The Britain squad are currently in a team meeting, with a statement expected at 0830 GMT.
Team officials say that no replacement will be called up for Long, who was outstanding in Britain's 23-12 win over Australia in Sydney last week.