David Lammy has hailed the decision to return the Chagos Islands to Mauritius as a deal to save a strategically important UK-US military base, after accusations from opposition MPs that a key asset was being given away.

The government announced last week that it was going to hand over the Chagos Islands to Mauritius, ending years of bitter dispute over Britain’s last African colony, but the military base on Diego Garcia will remain under UK control.

In a statement to parliament, the foreign secretary said the deal had to be agreed as the status quo was clearly not “sustainable”. Citing US support for the agreement, Lammy told MPs: “It’s critical for our national security. Without security of tenure there will be no base. The deal benefits us, the UK, the US and Mauritius.”

He added: “This is a victory for diplomacy. We saved the base, it has been secured for the long term.”

Lammy said the deal did not signify similar deals that would be in the offing for Gibraltar or the Falklands. “British sovereignty on the Falkland Islands, Gibraltar and the sovereign base areas is not up for negotiation,” he said. “The situations are not comparable. This has been acknowledged across our overseas territories.”

Five years ago the international court of justice issued an advisory opinion condemning the UK’s continued control over the Chagos Islands. Lammy said that if the government had not agreed the deal, a binding judgment against the UK seemed inevitable and it would risk losing the base or breaching international law.

Conservative MPs heckled Lammy over the deal on Monday despite the previous government having taken part in 11 rounds of negotiations, the last held just weeks before the general election. Negotiations were initiated in November 2022 and the former foreign secretaries James Cleverly and David Cameron took part.

The shadow foreign secretary, Andrew Mitchell, said that the government “proposes to give away a key military asset”, saying this “gives succour to our enemies in a dangerous world”.

The former immigration minister Robert Jenrick also condemned the deal, saying: “We’ve just handed sovereign British territory to a small island nation which is an ally of China, and we are paying for the privilege, all so the foreign secretary can feel good about himself at his next north London dinner party. In whose interests does he think he serves: those of the global diplomatic elite, or those of the British people and our national interest?”

Lammy said that in return for Mauritius having sovereignty over the islands including Diego Garcia, base operations will remain under UK control into the next century – initially for 99 years with the UK having a right to extend.

The agreement will allow a right of return for Chagossians, whom the UK expelled from their homes in the 1960s and 70s in what has been described as a crime against humanity and one of the most shameful episodes of postwar colonialism. “The manner in which Chagossians were forcibly removed in the 1960s was deeply wrong,” Lammy said.

Asked about the fate of more than 60 Tamil refugees stranded on the island of Diego Garcia for the last three years, Lammy said: “On signing the treaty this is now a matter for Mauritius.”

Yasmine Ahmed, the UK director of Human Rights Watch, which supported a protest by some Chagossians opposed to the deal outside parliament on Monday, said: “Chagossians have a right to be consulted about their homeland but they were not in fact consulted. The government must bring the Chagossians into these negotiations. They cannot negotiate away their responsibility towards the Chagossians.”