The first half of the high-speed Channel Tunnel Rail Link is due to open on Tuesday.
Eurostar trains will be able to run at their full speed of 186mph
The new section of line will cut journey times from London to Paris and Brussels by 20 minutes.
The £1.9bn, 46-mile first section runs from the tunnel near Folkestone, to
Fawkham Junction in north Kent.
The gentle curves and gradients and computerised signalling mean Eurostar trains on the line will finally be able to reach their full operating speeds - 186mph.
This will make journey times to Paris and Brussels 20 minutes shorter, with Paris just two hours 35 minutes
from London, and Brussels two hours 20 minutes.
Pip Dunn from Rail Magazine said that would be an invaluable saving for many business travellers.
"This will bring us into the premier league of high-speed railway, and hopefully it will prove to be the springboard for future projects such as the London-Scotland high speed route," he said.
NEWER FASTER TIMES
London-Paris: 2hr 35min
London-Brussels 2hr 20min
London-Lille: 1hr 40min
The £3.3bn second half of the link will run from north Kent to St Pancras, and will be completed in time for
passenger services to start in 2007.
When the 68-mile link is completed and runs all the way from London to the coast, the London-Paris time will be two hours 15 minutes and London-Brussels two hours.
The link will also provide the infrastructure for high-speed commuter services from Kent to London.
Many cross-Channel commuters will be relieved at the end to the "national embarrassment" of trains slowing sharply as they cross from France to England.
It is nine years since the UK's high speed route to the Channel was first decided, and 10 years since France completed its version.
Passenger services on the first section will start running at the end of the month.