A closure mechanism for a beverage preparation machine comprising a fixed lower part and an upper part rotatable relative to the lower part, the upper part being moveable from an open position, in which the upper part is disengaged from the lower part, and a closed position, in which the upper part is engaged with the lower part, the closure mechanism further comprising a clamping lever operable to retain the upper part in the closed position, wherein the clamping lever comprises a first member being pivotably connected at a first pivot to the upper part and a second member having a first end pivotable about a second pivot, an end of the first member and a second end of the second member being pivotably connected together at a third pivot such that the clamping lever has first and second positions of stability, wherein on operation of the clamping lever to move the upper part from the open position to the closed position the clamping lever moves from the first to the second position of stability.