Arlen cleared to face Simmonds
The showdown between Ellie Simmonds and Victoria Arlen in the S6 400 metres freestyle will go ahead after the International Paralympic Committee confirmed the American's eligibility to compete.
Earlier this week Arlen's participation had been thrown into doubt when she was deemed ineligible and excluded from London 2012 but the United States' appeal to the IPC was upheld and the 17-year-old was reinstated on Thursday.
In a two-stage process, Arlen passed an initial assessment but she was also observed during racing and following that, the IPC confirmed she was to continue to be classified as an S6 swimmer in her freestyle events with a review in a year's time.
A swimmer's classification can change for different strokes because the nature of their impairment may affect their ability to perform a particular stroke.
This applies to Arlen who competes in the SB5 class in breaststroke in which she will continue to be observed.
An IPC statement read: "IPC Swimming can confirm that following this morning's heat in the Aquatics Centre the USA's Victoria Arlen will continue to compete in the S6 class with a review date set for August 2013.
"Due to the different technicality of stroke required in breaststroke a similar observation in competition will take place in her first SB race."
For Arlen there will no doubt be enormous relief while the crowd at the Aquatics Centre could well be treated to a battle royale after the evidence of the heats.
Simmonds produced a blistering five minutes 24.64 seconds, just 0.18 seconds outside the world record Arlen set in June at the US trials.
The American then put the doubt surrounding her to one side to qualify second fastest in 5:25.19, 0.55secs behind Simmonds.