May informed MPs of the triggering of Article 50 earlier today, with a letter simultaneously dispatched to Tusk in Brussels.
And the Prime Minister struck a collaborative tone, repeatedly talking of a "deep and special" partnership with EU members, paying tribute to the "liberal democratic" values of the European Union, and stressing areas where the UK would continue to collaborate.
Labour MP and Treasury committee member Wes Streeting backed the shifting message from Downing Street.
"The biggest risk for both Britain and the EU is that politics trumps economics, security and other crucial areas of cooperation," he told City A.M.
"The tone that people on both sides of the channel need to adopt is one that recognises that is in neither of our interests for the UK to crash out with no deal. And the tone that the PM struck in the House of Commons is better than we have seen from many ministers in the past six months or so."
In particular, Streeting welcomed May's commitment to protecting "crucial" sectors like the City.
"I know it's fashionable to bash financial services, but as we have seen to our cost, Britain's economy may be over reliant on financial services but the fact is, it is heavily reliant on financial services.
He added: "I don't think anyone seriously believes that we are on course for a Banker's Brexit, but we need the PM going to Brussels to bat for financial services and other key industries."
Conservative MEP Syed Kamall told City A.M. her comments would likely be well-received in Brussels.
"She is being constructive and very clear about what she wants," Kamall said.
"This is a good start. Both sides realise that the mood music is important, and now negotiations have started people want to be as constructive as possible."
However, the Prime Minister's softer message was less evident in her letter to Tusk, in which she suggested that collaboration on security could fall by the way-side if the UK cannot secure a deal.
"In security terms a failure to reach agreement would mean our cooperation in the fight against crime and terrorism would be weakened. In this kind of scenario, both the United Kingdom and the European Union would of course cope with the change, but it is not the outcome that either side should seek," she said.
A Downing Street spokesman said May was simply "stating the facts", but Lib Dem leader Tim Farron accused May of delivering a "blatant threat".
“Security is too important to be used as a bargaining chip and this will backfire in any negotiations, which rather than building up alliances will leave Britain even more isolated,” Farron said.