Image caption Changes to funding and visa rules may mean UK universities struggle in next year's rankings

UK universities have taken four of the six top slots in a global university "league table".

Cambridge - top last year - was second in the QS World University Rankings, which are based on a number of factors.

UCL, Oxford and Imperial took fourth, fifth and sixth places respectively, with the US's Massachusetts Institute of Technology top and Harvard third.

But Ben Sowter, of QS Research, warned higher fees and new visa rules could see UK universities struggle next year.

Mr Sowter told BBC News students trying to get ahead of a trebling in fees for English undergraduates had produced a surge in applications to start university in autumn 2011.

He added: "We won't know for sure until the 2012 results come in, but we may see a drop off next year."

He also said tougher visa rules for international students could deter some from applying to UK universities.

QS RANKING: TOP 10 1. Massachusetts Institute of Technology (3)

2. University of Cambridge (1)

3. Harvard University (2)

4. UCL (7)

5. University of Oxford (5)

6. Imperial College London (6)

7. Yale University (4)

8. University of Chicago (8)

9. Princeton University (13)

10. California Institute of Technology (12)

He quoted a second piece of research to be published by QS later in September that asked employers across the world whether "current visa regulations adequately support the hiring of graduating international students".

Just 40% of UK employers agreed visa regulations had helped them hire international graduates - globally the figure was 70% said Mr Sowter.

Government attempts to tighten the student-visa system attracted criticism from two reports by MPs within the past week. Both argued overseas students should be reclassified so that they would not count towards migration limits, to allow the UK to continue to expand its share of the overseas student market.

The QS rankings rate the world's top 400 universities, evaluating each institution's strengths in research, teaching, the employability of its graduates and international outlook.

Dr Wendy Piatt, director general of the Russell Group of research-based universities, said: "With four institutions in the global top six and 18 in the top 100, our universities punch well above their weight, making the UK one of the few countries with a place at the top table - but the UK has already slipped below Mexico and Russia and has been caught up by India in terms of investment in higher education as a proportion of GDP [gross domestic product].

"If we are serious about staying on top, the government must concentrate investment where it will have the most impact - in our world-class research-intensive universities.

"These tables show that growing numbers of international students are a common feature among world-beating universities, with UK institutions leading the way.

"As it seeks to clamp down on abuses of the visa system, the government must be careful not to push the best and the brightest students away from the UK and into the arms of our competitors."

A Department for Business Industry and Skills spokeswoman said: "The government has put our university system on a stable footing by introducing substantial government-backed loans, which secure a sufficient stream of funding for the sector while ensuring there are no upfront fees and a progressive repayment system for students.

"We believe the attractiveness and quality of our higher education will be sustained by our reforms."