Ahead of Phillip Hammond’s Budget, General Sir Richard Barrons urged two billion pounds a year be allocated to the military otherwise, he claimed, it could simply "fall over".
Mark Francois, who served as a defence minister from 2012 to 2015, has also spoken out. He told the Mirror:
"We should never take living in a free country for granted and I believe the international situation, including Russia and North Korea, now justifies an increase in our defence expenditure."
Former Defence Secretary, Sir Malcolm Rifkind, echoed the warnings.
This time stating that if Britain wants to punch above its weight militarily, then it needs the resources to do so.
Concerns were also voiced by the former International Security Strategy Minister, Sir Gerald Howarth, as he described the world in a "critical condition".
"It's not just North Korea, the Chinese are basically colonising the whole of the South China Sea, with potential risks to our trading operations.
"We have got great uncertainty in Iran where they are completely immune to any concern about human rights and seeking to destabilise much of the Middle East, (and) we have Russia doing its level best to disrupt us through cyber and sabre-rattling on the borders of the Baltic states."
A spokesperson for the Ministry of Defence stated: “As one of the few countries to exceed NATO’s 2% spending target, our rising defence budget is the largest in Europe.
“We are investing in brand new equipment from submarines, frigates and carriers for a growing Navy, to state-of-the-art vehicles for the Army, to a whole range of cutting-edge technologies through our £800 million innovation fund.”
However, Sir Peter Luff, former Defence Equipment Minister, who served at the MoD between 2010 and 2012, warned the budget is over-stretched and unworkable in its current state.
"Defence inflation runs ahead of normal inflation, and soldiers, sailors and airmen deserve real terms pay increases, so the budget is under huge pressure.
"We can't on the current budget do everything we would like to do ourselves, we just can't."
It is believed the MoD has to make around £20 million of savings to plug a hole in its budget over the next decade.
There's been no indication yet from the Chancellor on what he plans to do about defence in Wednesday's budget.
In response to the letter, an MoD spokesperson said:
An MoD spokesperson said:
“In the face of rapidly intensifying threats, we are contributing to the cross-government review of national security capabilities and looking at how we best spend our rising defence budget to protect our country.
“We are investing in brand new equipment from submarines, frigates and carriers for a growing Navy, to state-of-the-art vehicles for the Army, to a whole range of cutting-edge technologies through our £800m innovation fund.”