Gaps between soldiers' tours are being extended by six months as the "relentless" pace of operations takes a toll on their marriages, the head of the Army said.
General Sir Richard Dannatt said servicemen and women had been "seriously stretched" by years of fighting in Iraq and Afghanistan, and would see time between overseas deployments rise to 30 months from two years as a result.
The Army was also "undermanned" and needs to be larger, he added, in what are being seen as his most outspoken comments yet.
In a speech to the Institute for Public Policy Research in London, Gen Dannatt said: "Many families and marriages have unfortunately fallen victim to the relentless pace of operations.
"We have seriously stretched our soldiers - both their goodwill and their families."
The general, who retires as Chief of the General Staff later this year, said he was reforming the Army's fighting brigades into larger units that can be sent away less often. This will see the ten brigades currently comprising around 4,000 men streamlined into six.
Gen Dannatt said: "A gap of one year between operational deployment is not unusual and often soldiers are spending much of the year before a deployment away from home, in training and preparation. This is unacceptable.
"The Army should be about 102,000 soldiers. It is currently about 98,500. By definition we are undermanned.
"There is a very strong argument for the size of our land forces to be larger."
He also criticised delays announced by the Government for the Army's next generation of armoured vehicles, saying much of what soldiers travelled in "harks back to the Sixties and Seventies".
Defence Secretary John Hutton, who is visiting troops in the southern Afghan province of Helmand, acknowledged that the decision to commit the UK to fighting in two theatres had imposed "very substantial strain and stress" on Armed Forces personnel and their families.
But he predicted that the expected withdrawal of most combat troops from Iraq later this year will reduce the burden by cutting the "operational tempo" experienced by the military.
Mr Hutton told the BBC Radio 4 Today programme: "I agree with what the General has said and I think we have accepted that the strain of mounting two major operations - one in Iraq and one in Afghanistan - is creating very substantial strain and stress on our military forces.
"The drawdown in Iraq that will take place later this year - the removal of most of the combat forces from Iraq - I think will give us an opportunity to take stock, to refresh and renew ourselves.
"But we are capable of mounting very substantial operations in Afghanistan - we have been for a considerable time - and we have got very significant resources committed to this campaign. We need to do that because this is vital for UK national security."
Mr Hutton said it was "not a regular occurrence" for troops to be sent on an operational deployment after a gap of as little as 12 months.
But he added: "There are some specialist units where there has been particular strain placed on them because they are small in number and we have got to address that."
Mr Hutton said: "I think the withdrawal of forces from Iraq will certainly allow us to create a better operational tempo in terms of the strain on servicemen and women and their families.
"When it comes to the reorganisation of particular fighting units, these are matters for the service chiefs to sort out."