Commons Speaker John Bercow has caused controversy by citing a 400-year-old rule which means Theresa May cannot bring her EU Withdrawal Agreement back before MPs unless it is substantially different from the one which was decisively defeated last week.

In his statement, the Speaker read out a section from Erskine May which states: “A motion or amendment which is the same, in substance, as a question which has been decided during a session may not be brought forward again during the same session… Whether the second motion is substantially the same as the first is finally a matter for the judgement of the Chair.”

READ MORE: What the papers say after John Bercow rules out third vote on Theresa May's deal 

But what is the Erskine May and what are the powers of the Speaker in the House of Commons? 

-What is the Erskine May?

The Erskine May is considered to be the most important works on parliamentary procedure and the constitutional conventions in the House. It is the authoritative book on parliamentary law and practice. 

The term 'Erskine May' is a shorthand way of referring to the Guide to Parliamentary Practice first published in 1844 by Thomas Erskine May.

It acts as a guideline to the UK's unwritten constitution and rules and offers insight into how the procedure in the Houses of Parliament has evolved and what conventions apply.

Since first published in 1844, it has grown from 496 pages to over 1,000 pages today.

This convention, which is quoted in Erskine May, actually dates back to 1604, before the guide was first published. 

- How has the current Speaker enforced the rules most recently?

Mr Bercow cited the Commons rulebook Erskine May setting out a convention dating back to 1604 that a defeated motion cannot be brought back in the same form during the course of a parliamentary session.

READ MORE:  Speaker Bercow 'sabotages' Theresa May's Brexit plan as Government left in further disarray by his intervention

His ruling, which came in an unexpected statement to the Commons on Monday, indicated that the Prime Minister cannot bring her EU Withdrawal Agreement back before MPs unless it is substantially different from the package which was decisively defeated last week.

- What is the role of the Speaker of the House of Commons?

The familiar shout of "Order, order!" explains part of the Speaker's job in Parliament. They are in charge of keeping order during debates and calling different MPs to take their turns speaking on various issues.

- What powers does the Speaker have?

The Speaker is the highest authority in the House of Commons and has the power to ensure MPs follow the rules, including asking them to be quiet while others are speaking, directing a member to withdraw remarks if they are deemed to be abusive, and suspending the sitting of the House if there is judged to be serious disorder. An MP can be suspended if they are felt to have been deliberately disobedient.

- Is the Speaker impartial?

The speaker is an elected MP, like all their colleagues in the House, and they are supposed to deal with constituency business as normal. However, when elected they are required to resign from their political party and to keep themselves separate from political issues, remaining impartial.

- How is the Speaker elected?

The current speaker was elected in 2009, using an exhaustive secret ballot system. MPs marked an X next to the candidate of their choice on a list. John Bercow got 322 of the 593 ballots cast, at which point the question proposing the successful candidate as Speaker was put before the House and he took the chair.