The Army will lose 17 major units as it cuts 20,000 regular soldiers by 2020, the defence secretary has said.
Philip Hammond told MPs that the units to go included four infantry battalions and two sections in the Armoured Corps.
He said the Army would be a "forward-looking, modern fighting machine", but Labour said the cuts were short-sighted and could put the UK at risk.
The number of regular soldiers is set to fall from 102,000 to 82,000, while reservists will double to 30,000.
The Army will be about half the size it was during the Cold War era - it had more than 163,000 troops in 1978.
'Flexible and agile'
In the infantry, Mr Hammond said, no current regiments or cap badges would be lost.
He said: "After inheriting a massive overspend from the last government, we have had to make tough decisions to implement our vision of a formidable, adaptable and flexible armed forces.
"After a decade of enduring operations, we need to transform the Army and build a balanced, capable and adaptable force ready to face the future.
"Army 2020 will create a more flexible and agile Army. Unlike the past, it will be set on a firm foundation of men and material, well trained, well equipped and fully funded.
"The regimental system will remain the bedrock of the Army's fighting future."
The four infantry battalions to disappear are the 2nd Battalion the Royal Regiment of Fusiliers, the 2nd Battalion the Yorkshire Regiment (Green Howards), the 3rd Battalion the Mercian Regiment and the 2nd Battalion the Royal Welsh.
A fifth infantry battalion, the 5th Battalion the Royal Regiment of Scotland (Argyll and Sutherland Highlanders), will become a single company to carry out public duties in Scotland.
The Armoured Corps will be reduced by two units with the mergers of the Queen's Royal Lancers and the 9th/12th Royal Lancers and the 1st and 2nd Tank Regiments.
The Royal Artillery, the Royal Engineers, the Army Air Corps, the Royal Logistic Corps, the Royal Electrical and Mechanical Engineers and the Royal Military Police will also be affected.
Shadow defence secretary Jim Murphy said: "New threats are emerging and weak and failing states outnumber strong by two to one.
"There's an arc of instability from west African states to central and south-east Asia. Non-state actors are on the rise, climate and population change are new sources of tension. The United States is pivoting towards the Pacific, while the European end of Nato will take greater strain."
The restructuring of the Army was drawn up under a plan by Lieutenant General Nick Carter, and referred to as Army 2020.
Details of the other changes are:
- The Royal Artillery will be reduced from 13 to 12 units with the withdrawal of the 39th Regiment Royal Artillery
- The Royal Engineers will be reduced from 14 to 11 units with the withdrawal of 24 and 28 Engineer Regiments and 67 Works Group
- The Army Air Corps will reduce from five to four units as 1 Regiment AAC merges with 9 Regiment AAC
- The Royal Logistic Corps will be reduced from 15 to 12 units with 1 and 2 Logistic Support Regiments withdrawn from the Order of Battle and 23 Pioneer Regiment disbanded
- The Royal Electrical and Mechanical Engineers will be reduced to seven units with the withdrawal of 101 Force Support Battalion
- 5 Regiment Royal Military Police will be removed
Ahead of the announcement, the former head of the Army, Lord Dannatt, said there could be as many as 11,000 compulsory redundancies.
Philippa Tuckman, a military injury specialist lawyer, said more care would need to be taken of the part-time soldiers.
"If the MoD is to rely more on the TA and reserves we need to be reassured that the support mechanisms and aftercare the MoD have in place for regular service personnel, including re-training to re-trade after an injury, are available to them as well."