"Finally, if we failed to agree on any next extension, there would be a risk of an accidental no-deal Brexit".
The EU will demand that Britain's extended stay in the bloc can not undermine its key institutions.
Among them are participating in the May 2019 European Union elections which the United Kingdom government has already put in place preparations for, and not reopening negotiations on the withdrawal deal Theresa May was given by the EU.
"The United Kingdom shall facilitate the achievement of the Union's tasks and refrain from any measure which could jeopardise the attainment of the Union's objectives", it added.
The council conclusions and Mr Tusk's letter also repeat that "in any event, the United Kingdom can revoke Article 50 at any time, as stated by the European Court of Justice".
So May will certainly be grilled by her European Union peers on how she'll deliver a negotiated Brexit if even her risky and late gamble of alienating her own colleagues to take tea with the enemy were to flop.
Instead, they said, the Brexit lag could go until the end of the year but the 27 national leaders in the bloc would have the final say on Wednesday. "This is why I believe we should also discuss an alternative, longer extension".
The law was passed on the initiative of MPs, subverting the normal order in which parliament debates and passes government-proposed legislation.
British PM Theresa May asks European Union for Brexit delay until June 30
Nigel Farage has announced that he will stand in the European Parliament elections next month for his new Brexit Party . The move comes after parliament has repeatedly been unable to find a solution to the UK's withdrawal from the EU.
"Neither side should be allowed to feel humiliated at any stage in this hard process".
"Any extension should serve a goal".
The British Prime Minister has now finished meeting with President Macron and spoke with Angela Merkel this morning. "Make the most of Europe's opportunities".
The EU Council President - who published his letter with a tweet stating "there are times when you need to give time time" - wrote: "We should treat the United Kingdom with the highest respect, as we want to remain friends and close partners, and as we will still need to agree on our future relations".
May is due to travel to Paris and Berlin on Tuesday to press her request for a short delay, before it is formally discussed by European Union leaders on Wednesday.
If EU leaders refuse to grant an extension when they meet in Brussels on Wednesday, Britain faces a chaotic "no-deal" departure from the bloc on Friday.
But EU government heads with whom power actually resides, and notably President Macron of France, legitimately fear that in practice a delay of a year would simply mean Brexit uncertainty squeezing out all other EU work till long after we're all dead.
Back at home, May's ministers are in crisis talks with the Labour Party to try to break the deadlock on Brexit, more than a week after the United Kingdom was originally supposed to have left the EU.