With almost all votes counted, option for full statehood was backed by more than 97% – but turnout for non-binding referendum stood at just 23%

A plebiscite in the US’s oldest and largest colony, Puerto Rico, has returned an overwhelming vote in favour of becoming the 51st state of the union, in a result instantly rejected as meaningless by opposition parties, which had boycotted the event.

With 95% of the vote counted, the option of formally joining the US as a fully fledged state was backed by 97.1% of those who cast their ballots. By contrast, full independence/“free association” with the US was supported by 1.5% and keeping the colonial status quo just 1.3%.

The ruling Partido Nuevo Progresista (PNP), which controls the Puerto Rican governorship as well as legislature and organised the referendum, claimed the result marked a resounding victory for statehood. Senior members of the party said that it would give the movement a huge boost as it sought to persuade Washington that after 119 years of colonisation it was time to bring Puerto Rico on board on an equal footing with the existing 50 states.

“This result is more than enough to take to Washington and urge Congress to do the right thing,” Luis Rivera Marín, the PNP’s secretary of state who oversaw the referendum, told the Guardian. He dismissed the boycott of the vote announced by the opposition parties as a sham.

“In the democratic process there is no such thing as a boycott,” he said. “In this system, if you don’t vote you don’t count.”

But the leading opposition groups dismissed the vote as an irrelevance. The Partido Popular Democrático, which broadly advocates the continuation of the current colonial arrangement whereby the island’s 3.4 million people have US citizenship but cannot vote in federal elections, said the ballot was flawed from the outset.

The party’s elections commissioner, Miguel Rios Torres, told the Guardian that the turnout had been so low as to make a mockery of the organiser’s claim that it represented the will of the Puerto Rican people. “This vote will do nothing to help our island, and the US Congress has promised nothing in terms of responding to it.”

The head of the Partido Independentista Puertorriqueno, Fernando Martín García, described the result as an “absolute, unmitigated disaster” for the movement pushing to become a US state. “They are going to make a feeble attempt to persuade Congress to listen, but Congress will see this flimsy result as an excuse to say it’s not enough.”

The independence party, which advocates a clean break with the US, had originally agreed to participate in the referendum. But it pulled out after the US Department of Justice insisted on including a colonial status quo option on the ballot form.

The official results recorded that just 23% of the island’s 2.3 million registered voters turned out to cast their ballot, compared with 55% in last November’s gubernatorial election won by the PNP candidate Ricardo Rosselló. PNP officials stressed that turnout is always lower in a non-general election year, but the figure is still likely to deflate the impact of the referendum.

In sheer numbers, the result is also likely to fall short of the dramatic push for statehood that the ruling party had been hoping for. The last time Puerto Ricans were asked in a plebiscite whether they wanted to become the 51st state, in 2012, some 830,000 said yes; this time that number had dropped to about 500,000.

Puerto Rico was handed over to the US at the end of the Spanish-American war in 1898 as a form of war booty. Since then it has laboured under the colonial relationship and in the past decade the island has endured a prolonged recession and accumulated debts and pensions shortfalls of more than $120bn.

Were Puerto Rico to be allowed to join the United States on equal terms, it would be the first to do so since Hawaii became the 50th state in August 1959. Given its population size, it could expect to gain the requisite two US senators, bringing the total in that chamber to 102, and five House representatives, which would need to increase its seats by law from the current 435.

But so far there are few signs of interest on Capitol Hill for effecting such a momentous change. When the previous plebiscite was held in 2012 , 61% of those who voted were in favour of statehood, but the reaction from Washington was a deafening silence.

This time around, though the proportion for statehood has risen dramatically there is the added factor of Donald Trump in the White House to consider. Trump repeatedly adopted a position hostile to Latinos on the campaign trail last year, including his threat to round up 11 million largely Hispanic undocumented immigrants and his promise to build a wall along the Mexican border.

Puerto Rico, were it to be given statehood, would be the only mainly Spanish-speaking state in the union.