England's exams watchdog has expressed "serious concerns" about the standard of some qualifications being offered to foreign students.
Two thirds of awarding organisations - including exam boards and specialist colleges - are offering courses which are below par, according to a review by Ofqual.
The regulator said it was taking immediate action against a number of bodies, including withdrawing the right of an organisation to award qualifications.
Ofqual's review looked at a sample of the degree or postgraduate-level equivalent courses offered to "Tier 4 visa" international students - those outside the EU who are paying for tuition in the UK - and to home students.
International student visas have proved controversial in the past due to allegations that they were abused by some to gain entry into the UK. As a result, the Government introduced measures to toughen up the system.
In 2012, almost 210,000 Tier 4 visas were issued, with 29 organisations recognised by Ofqual to provide courses that are equivalent to degree or post-graduate level, often in areas such as business, accounting and tourism.
The review was sparked by complaints about the the quality and validity of some post-graduate-level courses being offered by the Accrediting and Assessment Bureau for Post-Secondary Schools (AABPS), along with allegations of fraud and malpractice in providing qualifications to Tier 4 students.
It found that two thirds of the 13 awarding organisations included in the review had qualifications containing content and assessment that did not meet the standard required for courses of this level, while all of those taking part gave Ofqual examples of students' work which subject experts did not believe was up to scratch.
Two thirds of the student work examined by Ofqual was found to be below the level required.
Ofqual insisted that many bodies have already taken action to address concerns, and that in future, all of those providing qualifications to Tier 4 students will have to toughen up their systems for developing courses and assessment and moderating students' work as well as approving and monitoring colleges offering the courses.
The London Centre of Marketing (LCM) has already had its right to award qualifications withdrawn, the regulator said, and an notice of intention to withdraw this right has also been issued to AABPS.
Jane Farleigh, Ofqual's director of regulator operations, said: "Following complaints, we identified a number of potential risks around the Tier 4 market. This resulted in us launching a review into the level 6 and 7 qualifications offered to Tier 4 students.
"The review is not about the legality of students' entry in the UK. Our role is to make sure that the awarding organisations offering qualifications used in this sector are meeting the standards we require of them.
"We sampled a range of the awarding organisations, and found some serious concerns. As a result of which we have taken regulatory action against a number of awarding organisations including withdrawing recognition from LCM and publishing a notice of intention to withdraw recognition from AABPS. We are now going on to review the remainder of the awarding organisations offering these qualifications and will take action if we find cases where things are not up to standard."
She said that anyone taking a qualification offered by AABPS or LCM who has concerns should talk to their college.
"It is important that students are treated fairly and have access to high quality qualifications," Ms Farleigh said.
"Where this is not the case and students are being let down by qualifications that are not up to scratch we will take action."
A Home Office spokeswoman said: "The UK's education offer is one of the best in the world, and in order to retain that position it is vital that we regularly assess the quality of colleges and courses that are on offer. Courses and colleges that don't meet the criteria must face the consequences so that the UK can continue to be an attractive choice for international students."