Astana have been invited to take part in the 2008 Tour of Spain despite being banned from the Tour de France and the Giro d'Italia for doping offences.
However, High Road, the team formerly known as T-Mobile, are the only one of the 18 ProTour outfits not taking part.
Their team leader, Manx rider Mark Cavendish, said: "The organisers' decision is probably political."
Astana are one of 20 teams who have been invited to compete in the Vuelta, Spain's premier cycling race.
Organisers Unipublic did not give any reason for leaving out High Road.
German sponsor T-Mobile left cycling altogether, at the end of the 2007 season after a string of damaging doping revelations.
Last November, T-Mobile rider Patrik Sinkewitz was given a one-year ban for taking illegal substances. In June that year, several former T-Mobile riders, including Tour de France winner Bjarne Riis, admitted past doping offences.
"Perhaps the organisers want to please the crowd with more popular riders in their country"
The team was re-branded High Road, and includes some former T-Mobile riders such as Olympic medal hopefuls Cavendish and Britain's Bradley Wiggins.
Astana withdrew from the 2007 Tour de France after star rider Alexandre Vinokourov failed a doping test.
Team-mate Andrey Kashechkin failed an out-of-competition test a month later.
In 2006, several cyclists on the team - then known as Liberty Seguros - were kicked out on the eve of the Tour de France after being linked to the Operation Puerto blood-doping scandal in Spain.
But the new, overhauled Astana team have made much of their commitment to the fight against doping.
Cavendish tried to brush away his surprise at High Road's absence from the Vuelta, saying: "The race is not as important as the Tour de France and the Giro [d'Italia] for our sponsors.
"Actually, the Tour of Britain and the Tour of Missouri are more important to them than the Vuelta.
"Perhaps the organisers want to please the crowd with more popular riders in their country."
Unipublic said: "(All the teams) meet the maximum requirements in terms of ethics, image and administration."
Teams: Astana, Caisse D'Epargne, Euskaltel Euskadi, Saunier Duval, Andalucia-Caja Sur, Karpin Galicia, AG2r-La Mondiale, Bouygues Telecom, Cofidis, La Francaise des Jeux, Credit Agricole,
Gerolsteiner, Team Milram, Lampre, Liquigas, Quick Step, Silence-Lotto, Rabobank, Tinkoff, Team CSC.
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