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при первой возможности stresses
gen.	at the next opportunity (Andrey Truhachev); at the first opportunity (I will do it at the first opportunity.); as soon as ever I can (Anglophile); as accurate as possible (alenagorelkova); as soon as conveniently may be (oVoD); first thing (I can't help you now, but I'll do it first thing. 4uzhoj); upon availability (Bauirjan); when occasion offers (Alex_Odeychuk)
Игорь Миг	at the earliest; at the earliest moment; at the earliest possible juncture
bus.styl.	at your earliest convenience (Leonid Dzhepko)
busin.	as soon as possible (dimock); as soon as practicable (Alexander Matytsin); as soon as they become available (Alexander Matytsin); on a first-available basis (transler)
chat., email	ASAP (4uzhoj)
dipl.	when the opportunity presents itself (financial-engineer)
law	at the earliest possible opportunity (Alexander Matytsin); as promptly as possible (Alexander Matytsin); as promptly as available (Alexander Matytsin); as soon as reasonably practicable thereafter (Alexander Matytsin)
Makarov.	at the earliest opportunity
rhetor.	as soon as we can (Alex_Odeychuk)
отправить "при первой возможности"
avia.	FRAV (first available; код связи)
"отправить при первой возможности"
avia.	first available

при первой возможности: 31 phrases in 9 subjects

Banking	1
Business	5
Economy	1
Formal	1
General	16
Idiomatic	1
Law	2
Makarov	3
Military	1