An old rule means Bercow could take drastic action on Brexit

According to the Commons' rule book 'Erskine May', there is a clear precedent that a matter, once decided upon by MPs, cannot be considered again in the same session of parliament (which usually lasts a year - this current session has gone on for longer and will expire in the summer). Buried deep within on page 397, there lies: 'A motion or an amendment which is the same, in substance, as a question which has been decided during a session may not be brought forward again during that same session.' It continues: 'Whether the second motion is substantively the same as the first is a matter for the chair.' In principle then, it would be in John Bercow's gift to say that parliament has voted on a matter already and whether a second, third or fourth iteration is sufficiently different to merit further consideration.