Britain "may have no choice" but to introduce conscription as the country remains "absolutely not ready" to defend Europe against the threat from Russia, a former colonel says.
Downing Street said service would remain voluntary after the outgoing head of the British Army, General Sir Patrick Sanders, said earlier this week the UK should "train and equip" a "citizen army".
But Britain's former top NATO commander, General Sir Richard Sherriff, told Sky News a day later that it could be time to "think the unthinkable" and "look carefully at conscription".
On Sunday, former British Army Colonel Tim Collins agreed that it shouldn't be ruled out.
"The recruiting system that the armed forces has, has failed miserably," Mr Collins, who is a parliamentary candidate for the Ulster Unionist Party, told Sunday Morning with Trevor Phillips on Sky News.
"I think what we have to do is to wake up and realise that… Ukrainians are fighting for their liberty through the illegal invasion from Russia and if they do not win that, we are next.
"We are not sufficiently supporting the Ukrainians towards the victory that is essential to us as well as them.
"And, secondly, we're not making provision for the case where they would fail and they collapse and we are next - and we need to be doing both of those things."
He said he took a battalion as commanding officer that was "250 undermanned", but still managed to recruit "to full strength" within two years.
He said it "can be done" but current contracts have "clearly failed".
"And if they can't recruit, we'll have to go to conscription," he added.
"I'd prefer not to, but that's the only alternative."
Armed forces minister, James Heappey, said on Thursday talk of the UK introducing conscription if NATO goes to war with Russia is "nonsense".
Carlos Del Toro, the US navy secretary, has urged the UK to "reassess" the size of its armed forces given "the threats that exist today".
Downing Street defended the government's spending on defence, saying Britain has been Washington's "partner of choice" in its strikes against Houthi rebels in the Red Sea because of its "military strength".