These people will be the key players over the next few momentous days as the Prime Minister battles to keep her Brexit agenda, and her premiership, on track.
The PM backed-off from a crunch Commons vote on the EU Withdrawal Agreement in the face of a “significant” defeat.
Mrs May is now set to travel to Europe ahead of a pre-arranged EU summit on Thursday and Friday in a bid to get new reassurances over the backstop arrangements for the Irish border.
The PM’s future hangs in the balance as she faces rising criticism from both pro-and anti-Brexit wings of the Tory party over her handling of EU withdrawal.
The Labour leader is also under pressure from elements of his own party who want him to move a confidence vote against the Government.
Some 50 Labour MPs and peers have urged him to force the issue, as have the Liberal Democrats and SNP.
However, the Labour leadership has made it clear it wants to strike when it considers Mrs May to be at her weakest, and is, for now, keeping its powder dry until it sees what, if anything, the PM brings back from the EU.
I have decided to call #EUCO on #Brexit (Art. 50) on Thursday. We will not renegotiate the deal, including the backstop, but we are ready to discuss how to facilitate UK ratification. As time is running out, we will also discuss our preparedness for a no-deal scenario.
— Donald Tusk (@eucopresident) December 10, 2018
The European Council president said that Brexit had now been added to the two-day EU summit taking place this week after the events at Westminster.
However, Mr Tusk signalled the EU intends to stand firm, stating that the EU would not renegotiate the deal, or backstop measures on the Irish border issue, but would only discuss “how to facilitate UK ratification”.
Mr Tusk has repeatedly expressed sadness at the prospect of Brexit and its impact on both sides.
The European Commission president has also made clear that negotiations will not be re-opened.
Mr Juncker has made a point of saying the EU will stand firm with the Irish government over backstop arrangements for the Irish border which would see the UK remain subject to the bloc’s customs rules if no wider trade deal is agreed before the end of a transition period.
The Taoiseach has taken a much tougher stance on Brexit issues since taking over as Irish leader from Enda Kenny in 2017.
Heading a minority government and facing possible elections within the next few months, Mr Varadkar cannot afford to be seen to give ground to the UK at home.
Citing the need to maintain peace in Northern Ireland, Mr Varadkar has insisted that backstop measures must stay in place after the transition period “unless and until” a trade agreement is in place.
Just finished a call with the Prime Minister. My message was clear. The backstop must go. Too much time has been wasted. Need a better deal. Disappointed it has taken so long for Prime Minister to listen.
— Arlene Foster (@DUPleader) December 10, 2018
The DUP leader has been flexing her party’s political muscles over Brexit as Mrs May relies on it for a slender Commons majority.
Mrs Foster has insisted that the backstop measures are unacceptable and the DUP cannot support them.
However, the DUP has said it will back Mrs May if she faces a confidence vote in the Commons, but only if the EU Withdrawal Agreement is voted down or significantly changed.
The German Chancellor will be a major player in any moves on Brexit.
However, she is now a weakened figure after standing down as the leader of her CDU party, but remaining as Chancellor.
Mrs Merkel will be a key voice in any softening of the EU line, especially as French President Emmanuel Macron is expected to maintain a tough position.
The International Development Secretary, and prominent Brexiteer has sounded ambivalent about the Brexit deal at times.
At Westminster she is widely seen as the most likely next Cabinet resignation over Brexit.
Other leading Brexiteers, Environment Secretary Michael Gove, and International Trade Secretary Liam Fox, have – so far – publicly backed Mrs May’s stand.
Sir Graham Brady
The chairman of the powerful 1922 Committee of Tory backbenchers is the only person who knows how many MPs have put in letters calling for a vote of confidence in Mrs May.
If the figure reaches 48 Tory MPs then a vote would be automatically triggered.
Sir Graham said he backed the PM’s decision to defer the Brexit vote, but added: “I think it’s best to recognise we are in uncertain times. A very difficult point in British politics.”
The arch-Breexiter believes the chances of a no deal exit from the EU have increased due the Prime Minister’s Commons move.
The MP wants a “managed no deal” and has turned up the heat on Mrs May repeatedly insisting a new PM would be needed for such a course of action.
Mr Rees-Mogg was widely considered to have overplayed his hand recently when leading figures from the European Research Group of Tory MPs he heads called for backbenchers to put in letters into the 1922 Committee calling for a vote of confidence in Mrs May.