The group studied the NGC 3603 and RMC 136a clusters - regions of space where thick clouds of gas and dust are collapsing into even denser clumps.
In these places, huge stars ignite to burn brief but brilliant lives before exploding as supernovas to seed the Universe with heavy elements.
NGC 3603 is relatively close in cosmic terms - just 22,000 light-years distant. RMC 136a (more often nicknamed R136) is slightly further away, and is sited within one of our neighbouring galaxies, the Large Magellanic Cloud, some 165,000 light-years away.
The team found several stars with surface temperatures over 40,000 degrees - more than seven times hotter than our Sun.
The research shows these young stellar objects to be unbelievably bright, truly massive and also extremely wide - perhaps 30 times the radius of our Sun in the case of R136a1.
This artist's impression (L) shows the relative sizes (radius) of young stars, from low mass "yellow dwarfs" such as our Sun, through "blue dwarf" stars that are eight times more massive than the Sun, to a 300 solar-mass star like R136a1 (R). There are a number of low-density giants that are known to have an even bigger radius than R136a1
Up close the stars would look a mess, however. Unlike our Sun which appears as a defined disc on the sky, the giants identified by Professor Crowther and colleagues would be losing so much material through powerful winds from their puffed up atmospheres that they would have a fuzzy look about them.
One thing seems for sure - no planets would exist in orbit about them.
"Planets take longer to form than these stars take to live and die. Even if there were planets, there would be no astronomers on them because the night sky would be almost as bright as the day in these clusters," Professor Crowther joked.
"Some of these big stars are relatively close to each other, so even at 'night' you'd have another very bright star shining on you."
Previously observed giants had been seen to get as big as 150 times the mass of our Sun. The latest findings raise interesting questions about what the upper limits on size might be.
Ordinarily, there should come a point where the pressure from all the radiation emitted by a stellar behemoth pushes back against any further infall of gas and dust. In other words, there ought to be a physical barrier to excessive star growth.
But Professor Crowther adds a second factor - that of resource. There may not exist in today's Universe places that have sufficient supplies of gas and dust to feed ever more massive stars.
However, the new observations do give a tantalising glimpse of what the very early Universe might have been like. Many objects in the very first population of stars to shine shortly after the Big Bang are thought to have been monsters like R136a1.
When these objects blew apart, their cataclysmic demise was so violent they may not have left behind a remnant core of material as is often the case following a supernova; or even a black hole which is another common consequence, too.
Instead, these giants may simply have dumped all their contents back into space, dispersing heavy elements like iron equivalent to the mass of 10 of our Suns.
"The bigger picture to this research is that it gives us confidence that there were probably more of these really massive stars in much greater numbers early on in the Universe," Professor Crowther told BBC News.
The new results appear in a paper in the journal Monthly Notices of the Royal Astronomical Society.