"I'm intending to speak to President Trump about this matter," May told parliament, referring to Trump's stance on Jerusalem, which has sparked international cries of alarm.
"Our position has not changed. The status of Jerusalem should be determined as a negotiated settlement between the Israelis and the Palestinians and Jerusalem should be a shared capital," she said.
As he arrived for a NATO meeting in Brussels, Foreign Secretary Boris Johnson said earlier on Wednesday: "We view the reports that we have heard with concern."
World leaders react as Trump prepares to recognize Jerusalem as Israel's capital
Speaking later on the sidelines of the meeting alongside US Secretary of State Rex Tillerson, Johnson also said Trump's imminent decision underlined the urgency of a new US-led Middle East peace plan.
"We'll have to wait and see what the president says," Johnson said.
"But clearly this is a decision that makes it more important than ever that the long-awaited American proposals on the Middle East peace process are now brought forward and I would say that that should happen as a matter of priority.
"We would like to see, as a result of this, the Americans' proposals on the Middle East peace process brought forward," he added.
In recent months, Trump's son-in-law and senior adviser Jared Kushner has been working with a small team to develop a new US proposal to revive peace talks between Israel and the Palestinians.
But Trump's decision to pre-empt the process by backing Israel's claim on Jerusalem has triggered a chorus of international concern, amid fears that it could sink any hope of new peace talks.
As he waited for Johnson to arrive for a handshake photo opportunity, Tillerson told reporters that the Jerusalem issue had not overshadowed his meetings at NATO.
"We haven't had a lot of talks about that," he said. The British alarm follows stern criticism of Trump's proposal from EU diplomatic chief Federica Mogherini.
Both Israelis and Palestinians claim Jerusalem as their capital and previous peace plans have stumbled over debates on whether and how to divide sovereignty or oversee holy sites.
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