The declaration by 130 international representatives comprising the "Friends of Syria" group of nations warned that any use by Assad's government of chemical or biological weapons would be met by a "serious response".
The move comes after US President Barack Obama announced on TV this morning that Washington would also recognise the Syrian opposition.
"We've made a decision that the Syrian Opposition Coalition is now inclusive enough, is reflective and representative enough of the Syrian population, that we consider them the legitimate representative of the Syrian people in opposition to the Assad regime," Obama said in an interview with ABC News.
However, his announcement stopped short of authorising the United States to supply weapons to the rebels - something Obama has steadfastly refused to do.
"Participants acknowledge the National Coalition as the legitimate representative of the Syrian people and the umbrella organisation under which the Syrian opposition are gathering," said the draft declaration obtained before the meeting of major powers, excluding Russia and China, in Morocco today.
" Bashar al-Assad has lost legitimacy and should stand aside to allow "a sustainable political transition" process," said the text of the draft declaration obtained by Reuters.
Referring to recent Western intelligence reports suggesting that Assad could use chemical and biological weapons in the 20-month-old conflict, the major powers said "any use of chemical weapons in Syria would be abhorrent and that this would draw a serious response from the international community".
They also announced the creation of a relief fund "to support the Syrian people", calling on states and organisations to make contributions to the fund.