The European Union has demanded that Britain make "significant progress" on its divorce from the bloc before talks on a trade deal can start, in tough Brexit negotiating guidelines unveiled on Friday.
The European Union said it is willing to discuss a free trade deal with the United Kingdom but it is only going to finalize it once the country has officially left the bloc.
Mr Tusk will visit London for talks with the Prime Minister ahead of an April 29 summit in Brussels, when the guidelines will be debated and approved by the remaining 27 member states.
The outgoing French president stood firm with Merkel and the European Parliament who are seeking to ban any trade talks between the United Kingdom and the EU, until the Brexit process is over.
"We want to talk about fairness and commitment", he said.
"The British government has indicated that it will not seek to remain in the single market, but would like to pursue an ambitious free trade agreement with the European Union".
But the move, much desired by Downing Street, will only take place once "sufficient progress" has been achieved in talks on the arrangements for Britain's withdrawal.
The EU's opening gambit in what Mr Tusk said would at times be a "confrontational" negotiation with Mrs May's government also rammed home Brussels' insistence that while it was open to letting Britain retain some rights in the EU during a transition after 2019, it would do so only on its own terms.
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"The first phase of negotiations will aim to settle the disentanglement of the United Kingdom from the Union and from all the rights and obligations the United Kingdom derives from commitments undertaken as Member State", the document reads, and "provide as much clarity and legal certainty as possible to citizens, businesses, stakeholders and global partners on the immediate effects of the United Kingdom's withdrawal from the Union".
Tusk said the European Union will not punish Britain in the negotiations, saying that Brexit itself is "punitive enough".
Negotiations on future trade relations between the United Kingdom and EU could begin as early as this autumn, European Council president Donald Tusk has indicated.
"May very constructive generally, but there is also one threat in it", European Parliament Brexit co-ordinator Guy Verhoftstadt said, adding that May seemed to be demanding a good trade deal in exchange for continued security co-operation.
Key EU leaders such as German chancellor Angela Merkel, while sympathetic to Ireland's concerns, have to date been wary of the mandate highlighting specific national interests of any member states.
"That negotiation is led exclusively from the European side by [European Commission negotiator] Michel Barnier", he said, adding that this was a "clear demarcation line".
French President Francois Hollande has already told the British Prime Minister that the withdrawal agreement should come first.
The EU's draft letter states that Britain's exit bill will be a once only payment.