Theresa May’s historic Article 50 letter was hand delivered to Tusk in Brussels on Wednesday afternoon.
Article 50 of the Lisbon Treaty provides a path for the process by which the UK will exit the EU.
The European Council - representing the EU’s national governments – is expected to issue guidelines to serve as the basis for negotiations.
Some believe Britain's departure leaves a substantial hole in the EU which now loses the world's fifth-largest economy, a nuclear power and a member of the UN Security Council.
However, a draft European Council statement gives a useful insight into how tortuous and possibly acrimonious the upcoming Brexit negotiations could be.
The leaked statement, which is expected to form the basis of the EU’s guidelines for its chief negotiator Michel Barnier, says that EU member states “regret” that the UK will leave the Union, but “are ready for the process that now will have to follow”.
Barnier, a former EU commissioner and MEP, warned last week of “serious repercussions” for both Britain (and the EU) if the divorce talks fail without a deal being agreed.
With the start of the Brexit talks now set to begin, MEPs were quick to react, with Ashley Fox, leader of Britain’s Conservative MEPs in the European Parliament, saying, “I sense the overriding mood in Brussels is one of determination to strike a deal as quickly as possible that is fair to both sides and recognises we will continue to be close trading partners and allies after Brexit.”
"That said, it will not be easy and a spirit of compromise is going to be required. Let's now tone down the rhetoric and allow our negotiators to get on with the job. It helps no-one if the talks are accompanied by a chorus of political heckling from the sidelines.
"Throughout the process Conservative MEPs will continue to represent the UK's interests and do all we can to ensure the negotiations reach a successful conclusion, voting on the final deal when it is brought before Parliament. It is time to begin shaping a prosperous new future for our country."
Syed Kamall, who leads the European Conservatives and Reformists (ECR) Group that includes the British conservatives, said, "This is an important day for the UK and for the EU. I hope that all parties involved will treat the negotiations calmly, constructively and in a spirit of co-operation. We must all keep in mind that at the heart of these negotiations are people's lives and livelihoods.
"It must not be about winners and losers. A bad deal for one side will also be bad for the other. We should aim for an arrangement that safeguards trade and allows continued cooperation in key areas of mutual interest for our security and economies.
“The ECR believes that Brexit is also an excellent opportunity for genuine EU reform. The call for a more flexible Union which better serves the interests of its members and citizens has never been stronger and ECR will keep leading the movement for change.
"I will do all I can to help secure a fair agreement, acting as a bridge between key individuals in Brussels and the UK Government.”
"I am confident of the outcome and look forward to a future in which both the UK and the EU thrive as close friends, allies and trading partners."
Fellow British Conservative MEP Amjad Bashir said, "Now we can concentrate on the important work, which is to secure the best deal possible for Britain as we leave the EU - as well as work on prospective trade deals with countries around the globe.”
Further comment came from UK-based Open Europe, campaigners for reform of the EU, which says that leaving the EU’s Customs Union is the “only logical step” for the UK to pursue an independent trade policy and achieve a truly ‘Global Britain’ outside the EU.
Open Europe’s Policy Analyst, Aarti Shankar, said, “We have looked at the evidence and at international examples and conclude that leaving the EU’s Customs Union is the right decision for the UK. If the UK remained in the Customs Union after Brexit, it would not be able to meet the Government’s ambition of conducting an independent trade policy and achieving a truly ‘Global Britain’.”
The EU and UK now have a two year deadline in which to strike a deal, in other words, the terms governing the new post-Brexit relationship between the EU and UK of which a trade agreement is the critical centrepiece.
If no new deal is agreed before the UK leaves the EU, trading terms will default to the World Trade Organization (WTO) standards, which are significantly less beneficial for both sides.