An immigration row has erupted between Donald Trump's supporters over a long-standing US visa programme.

The feud is about H-1B visas, which allow US-based companies to bring in skilled workers from abroad into certain industries.

Some immigration hardliners say the scheme undercuts American workers - but proponents say the visas allow the US to attract the best expertise from around the world.

The president-elect has weighed in, saying he supports the programme - despite being critical of it in the past - and tech billionaire Elon Musk has also defended it, saying it attracts the "top ~0.1% of engineering talent".

Here's what the data tell us about who gets into the US on these visas.

The H-1B visas for skilled workers were introduced in 1990. They are typically granted for three years, but can be extended for up to six years.

Since 2004, the number of new H-1B visas issued has been capped at 85,000 per year - 20,000 of which are reserved for foreign students with master's degrees or higher from US universities.

However, that cap does not apply to some institutions such as universities, think tanks and other non-profit research groups, so more are often issued.

People can only apply for an H-1B visa if they have a job lined up with a US-based sponsor company or institution.

The US government also approves extensions for those already working in the country.

Just over 386,000 H-1B applications were approved in the 2023 fiscal year (October 2022-September 2023), the latest year we have full data for, according to US Citizenship and Immigration Services (USCIS) figures.

That includes almost 119,000 new H-1B visas and about 267,000 extensions to existing visas.

The 2023 total is down from more than 474,000 in 2022.

There have been efforts to restrict the H-1B further programme in the past.

In 2017, then-president Trump signed an executive order that increased scrutiny of H-1B visa applications. The order sought to enhance fraud detection within the scheme.

Rejection rates hit an all-time high under the first Trump administration, reaching 24% in the 2018 fiscal year, compared with rejection rates of between 5-8% under the Obama administration and between 2-4% under President Biden.

However, the total number of approved applicants under the Biden administration has been similar to that under Trump's first.

In the three years that followed President Trump's executive order (2018-2020), about 1.1 million applications were approved, with about 343,000 of those being first-time applicants.

In the first three years of the Biden administration (2021-2023), about 1.2 million applications were approved, with almost 375,000 being new applicants.

Demand often exceeds the amount of visas granted - in most years there are thousands more applications filed than approved.

In cases in which more applications are received than visas are available, US Citizenship and Immigration Services (USCIS) effectively runs the H-1B programme as a lottery - which detractors believe highlights a fundamental flaw in the system.

"Ultimately, if you're going to have a skilled worker programme for 'skilled' workers, you don't award these visas via a lottery," said Eric Ruark, the director of research at NumbersUSA, an organisation that advocates for tighter immigration controls.

"Obviously, that's not how you find the best and the brightest."

We don't have a full report on the 2024 numbers yet, but preliminary figures suggest applications have increased sharply.

The number of eligible registrations published by the USCIS showed 758,994 applications in the 2024 fiscal year, compared with 474,421 in 2023.

With Trump headed back to the White House in January, Mr Ruark said he believes that the resolution of the H-1B debate will ultimately be among the factors that defines his presidency.

"Is that second term going to be pro-American worker, or revert to the old establishment Republican position that immigration is designed to help employers - at the expense of American workers?" he said.

"That's going to be a huge fight in the second term."

The vast majority of approved applicants work in science, technology, engineering, and mathematics.

Most are in computer-related occupations - 65% in the 2023 fiscal year.

This was followed by architecture, engineering and surveying - about 10% of people approved in 2023 worked in those sectors.

In terms of companies, Amazon was the top employer of people on H-1B visas in 2024, hiring more than 13,000 staff via the scheme.

Other familiar names like Google, Meta, and Apple feature high on the employer list - ranking 4th, 6th and 8th respectively.

Tesla, one of the companies owned by Elon Musk - who has backed the programme - ranked 22nd, employing more than 1,700 people on an H-1B visa.

California and Texas were the states with the most people working on an H-1B visa in 2024.

The median yearly income of people working in the US on an H-1B visa in 2023 was $118,000 (£94,046).

The median yearly income for people in computer and mathematical occupations across the US is about $113,000 - slightly less than those in similar sectors via the H-1B programme.

The median household income in the US is about $60,000 per year.

While opponents of the H-1B system often make the argument that H-1B holders undercut the salaries of American workers, some immigration lawyers and experts push back on that notion.

The vast majority of H-1B holders earn more than the "prevailing wage" for their occupation - a Department of Labor-determined figure that calculates the average wage paid to similarly employed workers in a particular part of the country.

Shev Dalal-Dheini, senior director of government relations at the American Immigration Lawyers Association, told the BBC that, while prevailing wages "are not a full labour market test", they are indicative of the fact that H-1B visa holders aren't negatively affecting the rest of the workforce.

"Let's say you're a software engineer in Washington DC. You look at the going rate for software engineers in DC, and you have to certify that you're paying at least that amount," said Ms Dalal-Dheini, who also worked on H-1B issues while as an official at USCIS.

"You're not really undercutting wages that way."

Additionally, Ms Dalal-Dheini said that US firms must also pay significant fees to file H-1B petitions, often in addition to lawyer fees.

"Companies that end up sponsoring H-1B [recipients] are looking at costs of up to $5,000 to $10,000 in addition to what you would have to pay an American worker," she said.

"The bottom line is that if they could find an American worker that was qualified, most companies would probably choose to hire that American worker, because it would be a cost savings."

The vast majority of those approved come from India.

The latest data showed around 72% of visas were issued to Indian nationals, followed by 12% to Chinese citizens.

About 1% came from the Philippines, Canada and South Korea respectively.

About 70% of those who enter the US on H-1B visas are men, with the average age of those approved being around 33.

Additional reporting by Becky Dale.