Halliburton Co. and Baker Hughes Inc. called off their merger, once valued at nearly $35 billion, which encountered opposition on several continents from regulators who claimed that it would hurt competition in the oil-field services business.

The deal to combine the world’s second- and third-largest oil-field services firms after Schlumberger Ltd. appeared troubled since April 6, when the Justice Department filed a lawsuit to block it. The merger also had encountered opposition from regulators in Europe.

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