Aljaz Bedene will not be allowed to represent Great Britain in the Davis Cup after losing his appeal.
Bedene, 26, played three Davis Cup ties for Slovenia before becoming a British citizen in March 2015.
A rule preventing players representing a second nation was later introduced by the International Tennis Federation.
"I will have to assess my situation now before I decide on what steps to take next," said Bedene, who has lived in the United Kingdom since 2008.
Currently ranked 57th in the world, he gained UK citizenship in March and became the British number two behind Andy Murray.
He appealed against the ITF's decision on the grounds his passport application was lodged before the rule changed at the start of 2015.
Lawn Tennis Association chief executive Michael Downey said the British governing body was "very disappointed" by the outcome.
In November, Britain won the Davis Cup for the first time since 1936.
They beat Japan in the opening round earlier this month to reach the quarter-finals of this year's competition and face a trip to Serbia in July.
BBC Radio 5 live tennis correspondent Russell Fuller:
The decision has not come as a surprise as the new rule retains a lot of support within national governing bodies. If an exception had been made for Bedene, the principle may have been fatally undermined.
Daria Gavrilova has been allowed to play Fed Cup for her new nation Australia, but only because she had never represented Russia.
Bedene, frustrated with the handling of his case and the length of time it has taken, could still seek a ruling from the Court of Arbitration for Sport.