Shadow defence spokesman Liam Fox has expressed concern that British forces are being overstretched, as troops prepare for an Afghanistan deployment.
Royal Marine Commandos are the latest troops to head to Afghanistan
Royal Marine Commandos are due to fly out to Helmand province in Afghanistan - the first of more than 3,000 combat troops to join a Nato-led force.
With UK forces still committed to Iraq, Mr Fox said defence spending was down, recruitment was falling and morale low.
But a Ministry of Defence spokesman denied troops were overstretched.
To try and put the latest deployment in perspective, he said that out of a total of 200,000 UK troops, there were 8,500 in Iraq and a peak of 5,700 due in Afghanistan.
And the average time between tours of duty remained well within guidelines, he added.
Dr Fox said that while the government had increased the number of commitments for troops, defence spending as a percentage of GDP "has fallen from 2.7% when Labour came to power to just 2.2% now".
He also said recruitment in all three services had fallen in recent years.
"The consequence of all of these things put together - including what is happening in Iraq - is that the length of time between tours, that's the operational elements of the armed forces, has fallen."
Government guidelines stated deployments should be 24 months apart, but according to Mr Fox "the average for infantry units is now down to 21 months and we know that for certain elements of the Army the two intervals are now less than one year".
"Now that is having a major impact on morale and we have an overstretch in our armed forces that really needs to be dealt with."
'Itching to go'
But Commanding Officer of 42 Commando, Lt Col Ged Salzano, played down suggestions of overstretch and said his troops were "itching to get back out".
"Units of 3 Commando Brigade have not been deployed in operational theatres for 18 months or so," he said, a day before they fly out.
"The corps is not particularly stretched.
"The young men of 42 Commando are very much looking forward to this challenge. Eighteen months is about the right sort of time gap between major deployments."