Theresa May’s plans to prioritise immigrants’ skills, rather than their country of origin, in a post-Brexit shake-up of the visa system could bring numbers up, experts have warned.

Announcing the plan Tuesday, the Prime Minister insisted it would be the first time for decades the UK could set its own rules on migration and promised numbers would be bought down to “sustainable” levels.

The focus would be on tackling low-skilled immigration and more value would be placed on attracting skills desirable to British businesses, following a recommendation from the Migration Advisory Committee (MAC).

However, the new immigration rules will not be part of the trade discussions with the European Union (EU), as MAC recommended.

Meanwhile, Alp Mehmet, of the think tank Migration Watch, said the new system could “potentially increase [the numbers] by a substantial amount” because of loopholes including the removal of a cap on higher and medium paid migrants.

He told BBC News Tuesday morning: “We’re going to be drawing people from the rest of the world, the sort of lower skilled people, not the very highest, because inevitably business is going to be pressing for it because it’s cheap [labour].”

He pointed out the government has not specifically said what skills would be welcomed according to the new system. If the pool of skills is widened to include IT workers for example, as a report by MAC has suggested, numbers could increase, Mr Mehmet said.

"Free movement will end" and people will only be able to come to the UK based on the contribution they can make, says Prime Minister Theresa May, as she sets out her immigration plan post-Brexit #r4today pic.twitter.com/jgJgSFiPn7 — BBC Radio 4 Today (@BBCr4today) October 2, 2018

However, Mrs May said: “The new skills-based system will make sure low-skilled immigration is brought down and set the UK on the path to reduce immigration to sustainable levels, as we promised.”

She added: “At the same time we are training up British people for the skilled jobs of the future.”

The new policy, unveiled by home secretary Sajid Javid at the Conservative Party Conference Tuesday, will be covered in more detail in a forthcoming white paper this autumn.

The post-Brexit strategy will impose a minimum salary of around £30,000-a-year for workers wanting to stay for longer periods in an attempt to stop them competing with Brits for jobs.

Meanwhile, more highly skilled immigrants who secure jobs will be able to bring their families to the UK if they are sponsored by their future employers.

However, Mr Mehmet said the employers would have too much responsibility and that it would be “perfectly easy” for them to “manipulate the salary levels, as has been done already”.

'We need controlled migration – we've taken in over three million people which has caused problems,' says Iain Duncan-Smith.https://t.co/Vn8yAiwRem pic.twitter.com/fRpnxcVXuJ — Victoria Derbyshire (@VictoriaLIVE) October 2, 2018

On Monday, former Tory leader Iain Duncan Smith published a paper calling for rules meaning migrants must pay UK tax for five years before claiming benefits.

On Tuesday, he argued Mrs May’s plan would not stop the abuse of the system and predicted the UK benefits system would remain a “massive pull factor” for unskilled workers coming from the EU to compete with Britons.

Commenting on the MAC report last month, Lord Green of Deddington, Chairman of Migration Watch UK, blasted:

“This is a very technical report which seems blind to the impact of high levels of EU immigration on many communities in this country as a result of rapid population growth.

“With immigration adding one million to our population every three years this simply cannot go on. These proposals would permit continued high levels of immigration, including those with medium skills from all over the world.

“The overall outcome would be to weaken immigration control rather than strengthen it.”