"Let me be clear: you need to speak English to learn at our education establishments. If you can't, we won't give you a visa."

This was the stark warning issued by UK home secretary Theresa May in parliament last month as she unveiled tough new rules for student visas aimed at cutting the numbers of migrants using education as a back door into Britain.

May said the changes to the current Tier 4 student visa rules will target private education providers suspected of bending visa rules, reposition the UK as a destination for only the "best and brightest" scholars, and cut visa numbers by 80,000.

The rules, which come into effect from on 21 April, will require private education providers to gain Highly Trusted Sponsor (HTS) status before they can enrol adult students from outside the EU on long-term courses. To do that they must be vetted by one of five approved accrediting bodies.

Students must provide more secure proof that they have sufficient funds to support themselves before they are allowed to enter the country and they must be able to prove that their English meets minimum standards.

The measures have less direct impact on state-funded further education colleges and universities, which have won approval from ministers for their existing controls as well as tacit acknowledgement that revenue from foreign students cannot be jeopardised when other funding is being cut.

A major blow to private-sector colleges will be the withdrawal of their students' eligibility to work part-time while studying. Students at universities will continue to be able to work 20 hours a week, and those at state FE colleges for 10 hours, but those studying elsewhere will be barred from getting a job to ease living expenses.

For Britain's long-established English language schools sector, which is estimated to contribute $3bn to the UK economy, the rule changes will have immediate impact.

Nick Bray, managing director of the London Study Centre, said that it was unfair to discriminate against his students, many of whom were taking a year out of their university studies in their own countries to learn English in Britain. "While some will be able to study without having to work, the fact is that many of them want to work part-time, not just to earn some pocket money, but also to improve their English. Just because these students don't typically go on to university doesn't mean they aren't the 'best and brightest'," he said.

The UK Border Agency, which enforces visa policies, is withdrawing its recognition of Accreditation UK, the English language schools inspection scheme run by the British Council, and the Association of British Language Schools (ABLS), which together provide assurance of standards for much of the sector.

The new list of UKBA-approved accreditation bodies includes Ofsted, better known for inspecting state schools, and QAA, the standards agency for higher education. In a statement QAA said it is "not anticipating any involvement with English language colleges unless they also offer higher education – or a recognised access route to higher education." Ofsted said that any expansion of its remit would require "legislative change". Those schools that have already achieved HTS status from UKBA are waiting to learn when they must switch to an approved accreditation body, but those schools seeking HTS are also in the dark.

The director of a language school in north-west London, which currently has up to 250 students, and who asked to remain anonymous, said that confusion over accreditation status is now critical for his business. "We've been operating since 2008 and I want to apply for accreditation, but what is the point in spending $3,000 preparing for Accreditation UK if I am not going to be able to bring in foreign students? I contacted Ofsted last year, but they told me they didn't accredit language schools," he said.

The British Council said in a statement: "We will be working with the UKBA and the relevant education authorities to look at how we can continue to give quality assurance to students and their representatives."

According to Tony Millns, chief executive of English UK, the association that represents over 450 accredited English language teaching providers, only 20 of his members are without HTS status. More concerning is a limit on the number of Confirmation of Acceptance for Studies (CAS) letters, crucial for a student visa application, that schools will be allowed to issue before they achieve the new accreditation status. Millns says not all CAS letters result in students taking up courses, so the cap could constrain schools' revenues.

Millns rejects the claim that the government has lost trust in accredited English language providers. He points to a change in rules introduced in January that will allow EFL schools to enrol students for up to 11 months on the more easily obtained student visitor visa, which is otherwise valid for only six months.

But he does have concerns about further action if cuts in Tier 4 student numbers are not achieved. "The government is already talking about reviewing the criteria for HTS status. If they make it virtually impossible for schools to maintain HTS status, that really will be an attack on the sector."

But students who obtain the proof of funds and language ability under the new rules face another, final obstacle. Either on arrival or at visa-issuing offices, UKBA officers will have the power to bar students entry if they judge that their spoken English does not meet the minimum standards.

Even if the student has valid English language test certificates at B1 or B2 levels on the Common European Framework of Reference, depending on whether they want to pursue below- or above-degree level studies, an officer without training in assessing spoken English holds the key to their future.