With Tory leaders and strategists assembling in Cambridge for a weekend "brainstorming" session on the way ahead, Sir Leon used a speech to Tory European constituency chairmen and agents, at a Warwick conference, to reinforce the growing concerns of the Tory left.
The former Conservative Cabinet minister criticised Mr Hague's call for a referendum on the Amsterdam Treaty, and his unilateral decision to rule out membership of the single currency at the next election.
Sir Leon said there was no objective reason why the Tories should "remain agitated about Europe" and he said that if the party wanted to get back into office at the earliest opportunity, it should stop "sulking on the sidelines" and dump its obsession with Europe, "which has plagued the party for so long."
More specifically, however, he said that the new Treaty of Amsterdam was by no means a great leap towards European federalism.
"Claims that the present Amsterdam Treaty is a dramatic assault on British sovereignty do not seem to me to stand up to scrutiny," he said.
"So it is difficult to see the real case for a referendum on the Amsterdam Treaty."
Having denounced Mr Hague's referendum initiative, Sir Leon went on to attack the new leader's decision to rule out membership of a single currency for the lifetime of the next Parliament - possibly 10 years ahead.