Johaug's suspension for testing positive for anabolic steroid clostebol, while training in Italy in September 2015, is due to expire on November 18.

FIS are hoping, however, CAS will extend the Olympic gold medallist and seven-time world champion's ban as they claim the 13-month suspension, handed down by the Norwegian Olympic and Paralympic Committee and Confederation of Sports' (NIF) Adjudication Committee, was too lenient.

The FIS described it as "on the low end of the range of reasonable sanctions" when they filed their appeal in March.

Johaug arrived at the CAS headquarters in Lausanne this morning.

She was soon followed by manager Jørn Ernst, boyfriend Nils Jakob Skulstad Hoff and lawyer Christian B. Hjort.

"We feel well prepared," Ernst was reported as saying by Norwegian newspaper Dagbladet.

Hjort added: "We are well prepared and have gone through the issue with Therese yesterday.

"She is tense but composed."

Former Norwegian team doctor Fredrik Bendiksen and his lawyer Anders Schrøder Amundsen arrived shortly before Johaug.

Bendiksen, who claimed "full responsibility" for the Johaug incident and resigned from his post following the case, will testify in the hearing.

Although the hearing is today, it could be several months before the CAS announces its decision in the case. 

Should CAS rule in favour of the FIS, Johaug could face an extended ban which would put her chances of participating at Pyeongchang 2018 under serious threat.

The Games in South Korea are scheduled to take place from February 9 to 25.

Johaug has always denied wrongdoing and claimed to have used the banned substance inadvertently via a sun cream that was wrongly given to her by the team doctor.

Anti-Doping Norway argue she should be blamed for "not having undertaken further investigation of the drug she received and used", which reportedly came in a package marked with the word "doping".

They recommended a 14-month suspension before this was reduced by a month in the eventual NIF verdict.

The skier, who claimed she used the sun cream to treat cracked lips, insisted her doctor had given her the all-clear to apply it.

In April, Norwegian national team manager Vidar Löfshus promised Johaug a place in the squad when her suspension comes to an end.

Löfshus also said they were hopeful her ban would be "shortened rather than lengthened".

Johaug was a member of Norway's winning 4x5 kilometres relay team at the 2010 Winter Olympics in Vancouver.

She won 30km silver and 10km bronze at Sochi 2014.

The Norwegian has also won the overall FIS World Cup title twice.