The incident is said to have taken place at the meeting of leaders of the developed world in Canada, which was marred by an ongoing trade battle between the US and its European allies,
Ian Bremmer, president of political risk consultancy firm the Eurasia Group, claimed other leaders had pressed the president to sign a joint communique at the end of the summit.
Mr Trump reportedly agreed to sign the agreement, before another tense exchange took place between him and Ms Merkel.
“Trump was sitting there with his arms crossed clearly not liking the fact that he felt like they were ganging up on him. He eventually agreed to sign it,” Mr Bremmer told CBS This Morning on Wednesday.
“He stood up, he put his hand in his pocket and he took two Starburst candies out, threw them on the table and said: ‘here, Angela, don’t say I never give you anything.’”
Mr Trump eventually refused to sign the agreement, lashing out at the Canadian prime minister, Justin Trudeau, on Twitter and leaving the summit to fly to Singapore for talks with the North Korean leader, Kim Jong-un.
The US president and the German chancellor have maintained a difficult relationship since the former took office in January 2017.
Mr Trump angered Ms Merkel with his decision to pull out of the Iran nuclear deal and the Paris climate accord.
Her visit to Washington in April lasted little more than three hours, a stark contrast to the lavish state visit afforded to Emmanuel Macron, the French president, a week earlier.
“His personal relationship with Merkel is deeply broken,” Mr Bremmer added.
“The leaders obviously do not respect each other, they don’t think the other respects them and it’s pretty clear that they don’t.”
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