Donald Trump has strongly hinted that he will back a deal in which the UK hands sovereignty of the Chagos Islands to Mauritius, including the Diego Garcia military base, which is jointly used by the US.

“I think we’ll be inclined to go along with your country,” the US president told reporters during an impromptu press conference in the Oval Office with Keir Starmer, who is visiting Washington. He added: “I have a feeling it’s going to work out very well.”

The apparent backing may be a surprise to British officials, who had been prepared for possible pushback against the idea.

Speaking the day before, David Lammy, the UK foreign secretary, said Trump would have an effective veto over the plan, given the joint use of Diego Garcia.

The Conservatives have raised repeated objections to the draft plan to hand over control of the islands to Mauritius, with the Diego Garcia base remaining under UK control on a 99-year lease, and some in the government are also worried about the proposal.

Asked about the plan as he sat alongside Starmer, Trump said: “We’re going to have some discussions about that very soon and I have a feeling it’s going to work out very well. They’re talking about a very long-term, powerful lease, a very strong lease, about 140 years, actually. It’s a long time. I think we’ll be inclined to go along with your country.”

He added: “It’s a little bit early, we have to yet be given the details, but it doesn’t sound bad.”

Starmer has argued that the talks with Mauritius, started under the last government, are vital to secure the future of the military base, given uncertainty about the future of UK control of the islands after international court rulings.

But critics have questioned the need, with the Conservatives alleging the plan will see billions of pounds paid to Mauritius under the terms of the lease.

The UK maintained possession of the islands when Mauritius became independent in 1968. In the early 1970s, between 1,500 and 2,000 islanders were forcibly deported so that Diego Garcia could be leased to the US to use as an airbase. The proposed deal with Mauritius has also been criticised for having no real provision for them to return.

Speaking to ITV’s Peston show from Washington on Wednesday, Lammy had said the deal could be scuppered if Trump objected.

“If President Trump doesn’t like the deal, the deal will not go forward,” Lammy said. “The reason for that is because we have a shared military and intelligence interest with the United States, and of course they’ve got to be happy with the deal, or there is no deal.”