By Alastair Macdonald

BRUSSELS (Reuters) - As Britons prepare to vote in a June referendum on whether to stay in the European Union, some of the argument turns on the significance of a package of EU reforms secured by Prime Minister David Cameron.

Cameron insists he has negotiated a radical reshaping of Britain's place in the EU, a "special status". Critics at home, including members of his own Conservative government, say the deal was insubstantial -- a view echoed by some EU leaders.

As with much Brussels diplomacy, the negotiation involved not only narrowing differences so that all could agree on a compromise text but governments presenting the deal in often diametrically contradictory ways to their home audiences.

A senior diplomat from one of Britain's major EU partners poured scorn on the agreement, expressing the frustration many in Brussels feel at the effort spent solving what they view as an internal Conservative Party feud that has endangered the Union and which the deal may ultimately do little to resolve.

"This whole thing is just a repetitive series of selective tautologies," the senior diplomat said. "It's all in the treaties already ... Though we are not repeating the whole treaties, just those parts which are pleasing to British ears."

Examples of that in the agreement struck on Feb. 19 include the repetition of Britain's right not to adopt the euro currency or the assurance that a commitment to "ever closer union" in the EU treaties does not oblige Britain to give up more sovereignty.

But EU officials who worked to produce the compromise -- under the fearful pressure that failure could see the Union undermined by the departure of such an important member state -- reject the view that it is a mere political charade.

Real legal heft is to be found, they argue, in that very selectivity of the "repetitions and tautologies" noted by its detractors.

"Some have an interest in presenting it in that way," said a senior EU official involved in crafting the deal. "But (the description) is not adequate ... The jurisprudence covers a wide area ... This is a very selective repetition."

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Essentially, by emphasizing some parts of existing treaties and secondary EU legislation, as well as legal precedents set by past decisions of the European Court of Justice, EU leaders -- "the fathers and mothers of the treaty", in the official's words -- are giving future guidance for the constitutional judges.

"And so we change the course of the supertanker," the senior official told Reuters.

London-based think-tank Open Europe criticized the deal and EU leaders for failing to adopt more ambitious, EU-wide reforms.

However, directors Raoul Ruparel and Stephen Booth wrote: "It is not transformative, but neither is it trivial. It is the largest single shift in a member state's position within the EU."

The EU official said: "It is constructed by taking things from existing law. But the totality does change the course of the Union, and in that way it is helpful to the Brits.

"It does ... contribute substantially to changing the course of the Union in certain areas."