It is understood that Allen told the firm's management on Friday (28 January) that he was quitting over his lack of autonomy from the US management and the firm's stilted attempt at European expansion.
Allen first talked of his concerns with US management at a US committee meeting over a week ago.
Last week US managing partner Stephen Dannhauser and senior partner Tom Roberts arrived in London and addressed an emergency meeting in an attempt to appease fears over a split in management. But one insider says: "Morale is still at an all- time low."
Mike Francies, joint head of the London office at Weil Gotshal, says: "I think that Maurice has reached a mutual understanding with the firm and decided to leave."
He adds: "At any firm people have issues, Maurice has issues and I have issues."
He refused to say exactly what those issues are.
Francies says: "I am happy to say we have 100 top-class lawyers here and no firm of this size and stature relies on one person."
He adds that he is not aware of any other lawyers wishing to follow Allen and says he does not know what Allen intends to do next.