Shadow Defence Secretary John Healey told Forces News "size does matter" when it comes to the strength of the UK's Armed Forces.
His comments follow reports that next month's Integrated Defence Review will recommend major cuts to personnel and resources.
Mr Healey added he wants the Government to publish the Review "without delay" to "put an end to the confusion and speculation".
"Without it, the funding the Prime Minister announced [in November] remains funding without a strategy and without it, we've seen a series of leaks which are undermining the morale of forces personnel," he said.
"They undermine the confidence amongst our allies and, of course, they draw attention to our adversaries and some of our weaknesses."
Responding to reports the size of the army could be cut, Mr Healey told Forces News: "Our Armed Forces are already 10,000 below strength.
"It's hard to see how deeper cuts will do anything other than damage our ability simultaneously to deploy our forces and protect the country."
He added that in order to justify any cuts, the Government has got to show "we've seen a clear reduction in threats, or they're willing to reduce the type of operations and the role that our Armed Forces play" since the last Strategic and Defence Review five years ago.
"Otherwise, I have to say, it's going to be very hard to accept the case to see further personnel cuts."
In response to the latest speculation about the Integrated Review, the Army's Head of Strategy, Brigadier John Clark, said "how we compete and fight is changing all the time and the way of operating must change accordingly".
Major General (Ret'd) Tim Cross, a former commander of British personnel in Iraq and Kosovo also told Forces News he is worried the Army could be set to lose thousands of troops as a result of the Integrated Review.
He said he fears as many as 10,000 troops could go.
In December, it was announced the military would be making £1bn of cuts over the year in response to a defence funding black hole.
Mr Healey continued the greatest threat the UK faces at present "is the growing hostility and capacity of Russia" due to "the longer range missiles, the modernisation of their land and sea forces and their readiness to deploy more grey zone hostility directly to this country".
In terms of Britain's role on the global stage, Mr Healey said: "As the Biden administration starts to reset the US approach worldwide and deal as a big power with the challenge of China, it will require, and should expect, Britain to continue playing perhaps the leading role in the NATO alliance, and that's a commitment I make for Labour."
He added Labour's commitment to NATO is "unshakeable" and its commitment to the nuclear deterrent is "non-negotiable".
Mr Healey spoke to Forces News after setting out Labour's defence priorities in a speech at RUSI.
The Integrated Review is due to be published next month following delays caused by COVID-19, James Cleverly, Minister for the Middle East and North Africa has said.
It has been billed by the Government as the deepest and most radical re-evaluation of the UK's place in the world since the Cold War ended.