* May, Tusk opening gambits show will to compromise
* But both warned of collapse, that would dump UK out
* Negotiations proper to start in two months time
* Two year countdown to Brexit on March 30, 2019
By Alastair Macdonald
BRUSSELS, April 2 (Reuters) - The week Britain filed for divorce from the European Union brought relief in London and Brussels that crockery was not smashed - far from it - but negotiators fear tantrums or even a breakdown once talks start.
The EU's chief Brexit negotiator Michel Barnier sees them beginning in early June; first the 27 other member states must agree his mandate at an April 29 summit and ministerial meeting on May 22. But talks about talks - who speaks to whom, when, about what and even in what language - could start weeks earlier and may be no less fraught.
Two weeks ago, the Frenchman flashed aside his Parisian jacket to reveal a glimpse of a cudgel beneath; he drew the British a vision of supplies cut off and lines of trucks at the Channel port of Dover if they push their demands over the brink.
This past week was more genteel. British Prime Minister Theresa May and European Council President Donald Tusk's opening gambits for two years of negotiation left each side praising a "constructive" start. Yet both also communicated threats that a deal was not certain; Britain could still be dumped out of the EU into a legal swamp.
As the fateful day approached for May to trigger Article 50 of the EU treaty and launch an unprecedented exit that Brussels wants to ensure remains a one-off, officials on both sides privately sounded increasingly worried the process could work.
But as Tusk sent his draft negotiating guidelines to the 27 other member states on Friday, two days after May's letter triggering Article 50, a senior EU said: "We are more optimistic than we have been that we can get negotiations going."
Keen to stress negotiating red lines, Tusk insisted Britain must pay up a "Brexit bill", give rights to 3 million EU expatriates living in Britain and accept EU rules during years of transition to a new free trade relationship. Much British reaction, however, focused on a proposal to give Spain a veto on new trade terms with Gibraltar.
Setting aside grumbling over Britain's tiny enclave on the south coast of Spain, Iain Duncan Smith, a pro-Brexit former leader of May's Conservative party, told the BBC: "That statement by (Tusk) was a lot more reasonable than I actually had any expectation of."
A commitment to launch talks on a future free trade deal and deep partnership before details of the divorce settlement are reached was particularly welcome in London, even if it did not offer the immediate negotiations some in Britain called for.