By Jan Strupczewski and Alastair Macdonald
BRUSSELS (Reuters) - David Cameron has written to European Commission President Jean-Claude Juncker and his legal and negotiating team thanking them personally for their "creativity" in crafting last week's EU deal for Britain.
The warmth of the personal letter, seen by Reuters and sent on Wednesday after a summit accord to help the prime minister win a referendum to keep Britain in the bloc, contrasted with the acrimony that marked Cameron's vain effort to block other EU leaders from appointing Juncker chief executive 18 months ago.
It highlighted relief on both sides at finding compromises after Cameron's demands, notably to deny benefits to EU migrant workers, seemed to many constitutional lawyers to go beyond what was possible under treaties guaranteeing fundamental rights.
"I know very well how challenging this negotiation was and how much creativity and professionalism it demanded of you and your staff in the Commission," Cameron wrote from 10 Downing Street to 'Dear Jean-Claude'. "I am extremely grateful for your personal commitment to the success of the negotiation."
People present after the grueling summit concluded late last Friday in Brussels said Cameron seemed to go out of his way to find and thank backroom members of the EU negotiating teams.
"He seemed really pleased, and relieved," one EU official said. "So he should be. I wasn't sure we could do this."
A spokeswoman for Cameron confirmed he had sent letters to all 27 other national leaders as well as to Juncker, summit chair Donald Tusk and the president of the European Parliament.
"He expressed his thanks for their commitment to securing a successful outcome to the renegotiation," she said. The letters also confirmed he had now called the referendum for June 23.
Campaigners who want Britain to leave the European Union dismissed the package, which clarifies London's relations with the euro zone and gives it an "emergency brake" on EU immigration, as trivial.
Cameron says it gives Britain "special status" by explicitly exempting it from closer political integration, spelled out in legally binding guarantees of future treaty change.
"The contribution you personally, and the Commission collectively, made to this historic agreement was absolutely essential to its success," he wrote to Juncker, straying from protocol by naming officials by name for their contribution.
They included Juncker's chief-of-staff, the head of the Commission legal service, the head of its administration and Jonathan Faull, a British veteran of the EU executive.
Cameron signed off his one-page letter to the Commission chief by hand: "With thanks and best wishes, Yours, David".
In 2014, the Conservative leader denounced the former Luxembourg premier as too committed to a federal Europe to lead the Commission. The Sun newspaper labeled Juncker "the most dangerous man in Europe". Other British media offended him by alleging that he drank "cognac at breakfast".
But sources close to the negotiations since Cameron won re-election last year on a pledge to secure EU reforms say the two quickly patched up their differences after other leaders brushed aside Cameron's objections to appoint Juncker.
(Additional reporting by Kylie MacLellan in London; Writing by Alastair Macdonald; Editing by Dominic Evans)