Section 21(7)(a) in West Bengal Municipal Act, 1993
(a)one or more Councilors who are the members of such recognized political party have-(i)voluntarily given up his or their membership of such recognized political party, or(iA)[ joined another recognized political party, on] [Clause (iA) inserted by West Bengal Act 22 of 2000, w.e.f. 1.9.2000.](ii)have exercised the voting right contrary to the manner of voting of the majority of the Councilors set up by such recognized political party in the Municipality, or