The Court of Appeal has unanimously rejected attempts to overturn the ruling but the Government is seeking permission to appeal to the Supreme Court.
CLA President Harry Cotterell said: "Many CLA members lost out on the higher rate FIT payments when the Government illegally rushed through sudden payment cuts to the solar Feed-in Tariff.
"We regret the Government still intends to appeal even though two courts have now ruled it behaved unlawfully.
"We urge Ministers to stop wasting time and money on further appeals and stick to their own contingency plan. This will reduce the solar tariff for installations completed after 3 March and is the only way to end the uncertainty that is damaging investment."