A number of the UK's leading universities are sliding down international rankings, jeopardising their reputations as some of the best places in the world to study for a degree, according to research.
New tables show a widening gulf between a "golden triangle" of institutions in London, Oxford and Cambridge and the rest of the nation.
The UK has 10 entrants in the Times Higher World Reputation Rankings this year, up one from 2013 , but down from three years ago when there were 12 in the top 100.
Bristol University dropped out this year, while Leeds lost its top 100 place last year and Sheffield dropped out in 2012.
The rankings, which are based on reputation, reveal that the top spots are dominated by an elite group of "superbrands" that are recognised worldwide.
In first place was Harvard University, followed by the Massachusetts Institute of Technology and Stanford University.
Cambridge was the highest placed UK institution in fourth spot, down from third last year, followed by Oxford in fifth, down one from fourth.
Eight of the UK's 10 top 100 entrants come from the South East "golden triangle", the rankings show.
Besides Oxford and Cambridge, these were Imperial College London (up one place to 13th), the London School of Economics (up one to 24th), University College London (down five to 25th) and King's College London (43rd, up from the 61-70 band last year), as well as the London Business School and the London School of Hygiene and Tropical Medicine which both entered the table in the 91-100 band.
The two other UK universities in the top 100 were Manchester, which was 47th last year, and dropped to the 51-60 band this year, and Edinburgh in 46th place.
Rankings editor Phil Baty said: "The UK has lost three big-name universities from the list of the world's 100 most prestigious institutions since the rankings were first published in 2011. In 2012, the University of Sheffield exited the rankings, in 2013 the University of Leeds followed suit, and this year the University of Bristol misses out.
"Given how important global reputation is in attracting top international talent, collaborations and investment, this is cause for concern. The UK has some of the world's biggest university brands: we must protect them."
The United States has the most universities in the top 100, at 46, while Germany has six entrants, putting it just behind the UK.
Australia and Japan had five each, and 20 countries had at least one institution in the top 100.
Nicola Dandridge, chief executive of vice-chancellors' group Universities UK, said: "This table, and other rankings, suggest that the UK continues to possess, by some margin, one of the strongest university systems in the world.
"Rankings cannot provide a complete picture and this is just a small snapshot of our sector. University reputation is also very subjective.
"What is clear, however, is that if we want to maintain this leading position, we must start matching our competitors' increased investment in higher education. That is why, in the run-up to the 2015 general election, we will be calling on all parties to reveal how they plan to fund sustainably the world-class teaching and research in our universities."
Sally Hunt, general secretary of the University and College Union (UCU), said: "The UK continues to punch above its weight on the global stage, but as other countries invest in their universities and move up global league tables, we risk being overtaken.
"While these tables are at best only a partial guide to the strength of academic capacity, our higher education sector is a success story and ministers should be doing all they can to help it progress. Other countries recognise the value of their universities and if we do not match their investment we will struggle to maintain our position."
Wendy Piatt, director general of the Russell Group, which represents 24 leading universities, said: "Our leading universities continue to be held in high regard around the world with the UK coming second only to the United States overall. The UK was recently ranked as the best in the world for the quality of its research.
"However, UK public spending on higher education and research is comparable with that of Slovakia and Chile, far less than competitors like the US, China and most western European countries. And while we currently punch above our weight, only with increased, long-term investment in research will the UK's leading universities and hi-tech industries continue to compete on the world stage.
"Ranking universities is a process fraught with difficulties so students should not use league tables alone when picking a degree course."
A Business Department spokeswoman said: "The UK has a global reputation for excellence in higher education. We have strong institutions, a world-class research base and dedicated staff. To stay ahead in the global race, we are protecting the research budget, making UK research more accessible and delivering a better student experience."
Education Secretary Michael Russell MSP said: "Today's publication again highlights the global reputation of Scottish universities.
"Congratulations to the staff, students and management of the University of Edinburgh for maintaining their strong position in the world rankings and proving once again that Scotland is a desirable place to study."