Mr Cook told a fringe meeting at the conference that the current voting system prevented the party from campaigning honestly on behalf of the poor.
As it emerged that a behind-the-scenes deal has been devised to spare the Prime Minister an embarrassing vote on the issue this Thursday, Mr Cook was joined by Northern Ireland Secretary Mo Mowlam to deliver a strong message that Labour should back reform.
In what will be seen by his opponents as a coded attack on Tony Blair's ruthless wooing of the middle classes, he said that the first-past-the- post system "submerged" the marginalised in society from the political process.
"We shape our policy, tailored to the last swing voter, irrespective of the votes or views of those people who supported us through the preceding 20 years.
"Personally, I would much rather have a situation in which we can go out honestly and campaign frankly for what we believe in.
"The pitch of all the main parties at the last election was a pitch aimed at the voter with a steady income, probably with private pension rights, almost certainly with the ability to own their home.
"Those are the people that matter under the first-past-the post system. There is a terrible pressure under the first-past-the-post system for a homogenised, pasteurised politics which caters for the norm."
Mr Cook told the meeting, which was organised by pro-reform group Make Votes Count, that a referendum should take place before the next election. There was no evidence that there was any retreat on electoral reform among the party's grass roots, he said.
He added that the anti-PR lobby could only be defeated if campaigners for reform backed whatever proposals Lord Jenkins recommended next month, even if they were not perfect.
"We can't afford the luxury of having an alternative vote among ourselves over whether Jenkins has got it right," he said.
However, it has emerged that leading PR opponents have decided that a vote could prove too damaging to the leadership and will instead remit their emergency motions after the debate.
Mr Cook's speech underlined the need to avoid a conference decision on the issue after a day in which the depth of division among Government ministers was laid bare. Mo Mowlam also gave strong backing to PR and claimed that a referendum should be held before the next election.
Yet earlier in the day, Cabinet "enforcer" Jack Cunningham denied there was any commitment to hold a poll on PR this Parliament.
Foreign Office minister, Derek Fatchett and Defence Minister John Spellar went even further and launched formally the First Past the Post group of Labour MPs with vitriolic attacks on pro-reform campaigners and the Liberal Democrats. The anti-reform campaigners admitted that they were negotiating with the ruling National Executive Committee to remit the motion.Reuse content