DEF chiefs facing a troop crisis allow dismissed units to rejoin.
Those fired for stealing, fighting, failing a drug test or going Awol will be reconsidered.
Soldiers discharged for physical or mental health reasons could be let back — and keep any compensation.
The offer also applies to those who have been laid off.
But those discharged for taking heroin or selling drugs to other soldiers remain barred.
The Army website states that each reinstatement will be considered on a case-by-case basis.
They are 3,000 soldiers short and more are leaving than joining.
Defense chiefs spent more than £70m on recruitment from 2019-21.
And the reunification age was recently raised to 57 from 52.
But former military intelligence officer Colonel Philip Ingram said the move “reeks of desperation”.
He added: “The Foreign Office needs to look at why it is so difficult to keep people.
“Poor food, housing, underserved garrison towns and the need to supplement higher paid civil servants will all be factors.”
The Army insisted: “We continue to have sufficient numbers to meet operational needs.”