Labour plans to vote against the government's Commons motion due for debate tomorrow, which would approve military action in response to the alleged use of chemical weapons by the Syrian regime.
A senior Labour source told The Huffington Post UK that "as it's framed at the moment" the government motion - which is yet to be published - was too broad and could not be supported.
After a series of meetings with senior colleagues, Ed Miliband confirmed on Twitter on Wednesday afternoon that he intended to table his own amendment demanding action not be taken before UN weapons inspectors on the ground in Damascus have had time to report.
"Parliament must tomorrow agree criteria for action, not write a blank cheque," he said.
The decision by Labour to vote against the government will make it much harder for David Cameron to win the emergency vote given the significant number of Tory backbenchers who have expressed concerns about any intervention. The votes of traditionally anti-war Lib Dem MPs could prove decisive.
The senior Labour source told HuffPost UK: "The Lib Dems have to decide what side they are on."