Tony Blair was warned in a secret document of "long-term damage" to the armed forces through Britain's commitment to the Iraq War.
On the eve of the invasion in March 2003, foreign secretary Jack Straw and defence secretary Geoff Hoon told the prime minister the country had to cut its force levels by two-thirds by the autumn.
Mr Straw and Mr Hoon said keeping more troops in Iraq would be outside the Ministry of Defence's planning assumptions and would have an impact on other operations.
An extract from a classified letter from the Cabinet ministers to Mr Blair, dated March 19, 2003, was released today by the Iraq Inquiry.
Mr Straw and Mr Hoon wrote: "It will be necessary to draw down our current commitment to nearer a third by no later than autumn in order to avoid long-term damage to the armed forces.
"Keeping more forces in Iraq would be outside our current defence planning assumptions.
"If ministers wanted us to, we would need decisions now so that we would be able to recommend what would have to give elsewhere.
"Scaling down to nearer a third will limit our contribution thereafter to a maximum of around one brigade, a two-star headquarters and possible (sic) a contribution to higher level command and control, air and maritime components and support enablers.
"Our view is that we should probably agree now to tell the US, for planning purposes, that this is the upper limit of our contribution."
The UK's commitment to the Iraq campaign peaked at 46,000 troops in March and April 2003 and fell to 18,000 by the end of May, according to MoD figures.
