at your earliest convenience
From Longman Dictionary of Contemporary Englishat your earliest convenienceat your earliest convenienceformalTCMSOON as soon as possible – used in letters We should be grateful if you would reply at your earliest convenience. → convenienceExamples from the Corpusat your earliest convenience• Could you therefore please telephone me at your earliest convenience?• We should be grateful if you would reply at your earliest convenience.• I also enclose a Medical Assessment Card which you should complete and return at your earliest convenience.