Astronomers hoped that ISON would put on a spectacular show - but now say the comet could fizzle out before it completes its trip.
A comet which is hurtling towards the sun may not live up to its expectations of becoming one of the "brightest for a century", say experts.
Comet ISON is due to graze the surface of the sun at the end of the month and astronomers were hoping it would put on a dazzling display.
Earlier this week, Nasa said the sungrazer "will likely become one of the finest comets in many years" and could be the "brightest for a century".
But some scientists say that it could break up and evaporate before it completes its trip.
Josh Barker, from the national space centre in Leicester, said the sun threatens to destroy ISON with its gravitational pull and solar energy.
"It could be the comet of the century, if all goes to plan if all the little pieces line up just properly we could have one of the brightest comets that we've seen certainly in current human life and one of the brightest in history, so it could potentially be very exciting although there's a lot of margin for error."
Experts say a break-up of the comet would pose no threat to Earth because the fragments would continue to follow the same trajectory the comet was heading in.
On November 28, it will pass within 724,000 miles of the sun's surface when it reaches perihelion (the point when it is at its closest to the sun).
Ison is thought to be a few miles in diameter and will be travelling at 845,000mph.
The sun's rays will heat the ball of ice, metal and rock to 2,760C meaning it should be able to be seen with the naked eye anywhere from late November until the middle of January.
Among them is Dr Jo Jarvis, from Rugby, who said: "When you get things like comets coming through the solar system, they're something new and interesting to look at, something you don't get to see very often.
"With comets like this everyone is kind of on the edge of their seat because you kind of bond to it a little bit. You really want it to survive that trip round the sun."
ISON was discovered on September 21, 2012, by Vitali Nevski and Artyom Novichonok, two Russian astronomers of the International Scientific Optical Network (Ison), which is how it got its name.
The best time to view it will be in the morning before sunrise on the Eastern horizon.