History would not be kind to Chancellor Philip Hammond if he oversaw a "catastrophic reduction" in Britain's defence capability, a former head of the Army has said.
General Lord Dannatt was reacting to reports that elite units of paratroopers and Royal Marines could be combined to save money as part of major cuts to Britain's military.
The proposals - which included one which would see a cut in the armed forces' strength by more than 14,000 - were published in The Times on Friday and were dismissed by the Ministry of Defence (MoD) as "pure speculation".
The suggested cuts would reportedly see the loss of 11,000 soldiers, 2,000 Royal Marines and sailors, and 1,250 airmen.
A letter by General Dannatt published in the paper on Saturday said: "The reported cuts to our defence capability represent an appalling reduction in our national ability to defend our sovereignty, protect our citizens at home and abroad and uphold our values.
"The Ministry of Defence may dismiss The Times reports as merely options under consideration, but options can quickly become decisions - in this case disastrous ones.
"I wish Gavin Williamson all the very best in his vital battle with Philip Hammond and the Treasury.
"Mr Hammond, as a previous defence secretary, oversaw a catastrophic reduction in defence capability during his tenure at the MoD - he must not be allowed to do it a second time. History would not judge him kindly."
Mr Hammond was defence secretary when the 2010 Defence Review was being implemented - which saw numbers of soldiers, sailors and airmen cut.
On Friday, a Downing Street spokesman said Mrs May is still committed to not reducing the overall size of the armed forces.
"The commitment remains, but we have a review, and I'm not going to speculate on the details of the review," he told a regular Westminster briefing.
He added: "We are one of the very few countries to not only meet but exceed Nato's 2% defence spending target.
"We are committed to increasing the defence budget by at least 0.5% above inflation every year for the rest of the Parliament.
"As you know, we are looking at how we most effectively spend that rising defence budget.
"No decisions on the review have yet been made, so any speculation is purely that."