The number of British troops in Afghanistan could substantially increase under plans to bolster efforts to counter the Taliban.
Defence Secretary Gavin Williamson is understood to have written to the Prime Minister recommending the uplift, although a decision is yet to be made.
The Ministry of Defence said the UK’s contribution to the mission was kept “under constant review”.
Between 400 to 450 soldiers could be sent to the country to join the roughly 600 already deployed in non-combat roles following pressure from US President Donald Trump for international allies to do more.
But former general Sir Richard Barrons warned that even more troops could be required – and they should be prepared to accompany Afghan forces on operations.
The retired officer said the decision to end combat operations in Afghanistan at the end of 2014 “hasn’t worked”.
He told BBC Radio 4’s Today that 400 extra troops would “send an important message to our allies” that they should do more and would also signal to the Taliban “they will never bring this fight in Afghanistan to an end by fighting, they have to resort to dialogue”.
But he suggested a greater military presence and an enhanced – and more dangerous – role should be considered, with troops leaving the relative safety of their bases to accompany Afghan forces on operations.
“I absolutely think, if we are going to make a meaningful contribution, we will have to find the courage to train, advise, assist and accompany them into action,” Gen Barrons said.
“With that comes some risks and consequences, but that’s how you make a difference.”
Fmr senior UK army commander, General Sir Richard Barrons, says the decision to leave Afghanistan in 2014 hasn't worked: "when we left, it was not the case that the Afghan army was strong enough to tip the balance against the Taliban...that now has to be reset." #r4today pic.twitter.com/Eas9GEq3rb— BBC Radio 4 Today (@BBCr4today) May 18, 2018
The US has increased its presence in Afghanistan since Mr Trump unveiled his South Asia strategy in August last year.
Around 4,000 troops are thought to have been added to the 8,400-strong US deployment in the country, to bolster the Kabul government, train Afghan forces and fight the Taliban and other militant groups.
Gen Barrons suggested that the UK should deploy around 10% of the troops sent by the US, which would lift the British contribution to more than 1,200.
A Ministry of Defence spokesman said: “The support the UK provides Afghanistan on security, development and governance is crucial to building a stable state and reducing the terrorist threat to the UK.
“We remain committed to Nato’s non-combat Resolute Support mission, in which we play an important role, and keep our contribution under constant review.”
The military uplift is intended to prevent a Taliban takeover of the country, target Islamic State and al Qaida.
On Thursday, US State Department spokeswoman Heather Nauert said secretary of state Mike Pompeo had “reinforced our enduring investment in Afghanistan” in a conversation with the country’s president, Ashraf Ghani.
The reported change in approach is said to come amid concerns over how Mr Trump will approach a coming meeting of Nato leaders.
Theresa May is expected to join the president at the summit in Brussels in July, with Mr Trump then set to make a visit to the UK.
Mr Trump has repeatedly called for Nato members to meet their defence spending commitment of 2% of gross domestic product (GDP) and has threatened to walk away from the organisation if his demands are not met.
During a meeting with Mr Trump in Washington on Thursday, Nato general secretary Jens Stoltenberg said all members have increased their spending, in part due to Mr Trump’s approach.
After the talks Mr Stoltenberg said it is “very important that we all contribute more to our shared security”.