Liberal Democrats leader Sir Menzies Campbell said he would support plans to send more UK troops to Afghanistan.
Sir Menzies said the deployment "cannot be allowed to fail"
He said there was "no option" but to send more because current troop numbers were not enough to complete the job.
Former British Army Colonel Tim Collins said the UK military effort in Afghanistan lacked "a staged plan" and was too small.
The government is expected to announce it will send more troops to bolster forces already in the country.
The Ministry of Defence said the mission in Afghanistan was "crystal clear".
Sir Menzies told Sky News's Sunday Live: "It's become increasingly clear that the number of troops deployed there is not adequate to meet the task."
"This is a deployment which cannot be allowed to fail... if this were to fall apart, then it would be deeply, deeply damaging to the stability of Afghanistan and it would also be deeply damaging to the credibility of NATO," he added.
The Lib Dem leader said he did not know if the government had "ticked all the boxes" required to make the peacekeeping and rebuilding mission a success.
Col Tim Collins told the BBC's Sunday AM programme: "The British government has no idea what it wants to do.
"It invited the Army to go to Afghanistan and do stuff... (and) they have arrived there and discovered that the force package is too small."
In the last month, six UK soldiers have died in Afghanistan. An announcement on reinforcements is expected next week.
Col Collins - who made headlines with a stirring eve-of-battle speech in Iraq in March 2003 but quit the Army in 2004 - said Afghanistan was "a war we cannot shy away from".
"We must fight this war, we must win this one. Unlike Iraq, that's an optional one. There is no choice in this."
Col Collins also said a proper plan was needed to tackle Afghanistan's poppy crop.
About 90% of the world's opium - the raw material for the drug heroin - comes from Afghanistan and it is an important part of the country's economy.
Afghanistan's poppy harvest was, Col Collins said, a "fact of life" and attempts to persuade farmers to abandon their crops was "unrealistic."
Any plan needed "to replace the poppy crop with something that's sustainable," he said.
He said: "At this current time there's a shortage of opium on the world pharmaceutical market, so one option would be, in the short term, to buy the crop and use it for pharmaceuticals.
"And then with a staged plan - and this is what's missing every time - a plan to replace the poppy crop with something which is sustainable for the farmers.
"But just taking away their livelihood is just not an option."
Col Collins also said more troops should be sent to the region - but said that with existing deployments to Iraq and Afghanistan, there might not be any to spare.
A spokeswoman for the MoD said: "The international community's mission in Afghanistan is crystal clear. Put simply, Afghanistan must never again become a safe haven for terrorists.
Tim Collins says the military effort in Afghanistan lacks planning
"We must build security and government institutions so that the progress of recent years becomes irreversible, and combat the narcotics trade, the Taliban insurgency and illegally armed groups, all of which remain threats to Afghan security and stability.
"The Helmand Task Force has not been deployed to conduct counter-narcotics operations.
"Of course, ending the opium industry is essential for Afghanistan's long-term future.
Prime Minister Tony Blair has said that the mission of UK troops in Afghanistan was vital for the security of the UK and the wider world.
He said British troops were doing "the most extraordinary and heroic job", which was important for the security of the wider-world and vital "to our security here in this country".