The People's Vote campaign, which wants a new referendum on Brexit, is attempting to change Labour Party policy, according to a leaked memo.
It wants MPs and activists to submit a motion at the Labour conference next month, committing the party to backing a new referendum on the final deal.
It stresses that it is not an attempt to change the party leadership too.
Campaign leaders think the best way to get a new referendum is to get the main opposition party to back it officially.
At the weekend, the People's Vote commissioned polling which suggested voters targeted by Labour were more likely to back a referendum - and to favour remaining in the EU - than the electorate as a whole. It hopes that this might encourage a change in approach by the party's leadership.
The party's current policy position is to respect the result of the 2016 Brexit referendum, and not to call for a new one but to "leave all options on the table" if a deal is not agreed by Parliament. Those options would include a new election, and what shadow chancellor John McDonnell calls "democratic engagement", which might mean a further referendum if there is deadlock.
The People's Vote campaign believes a general election is unlikely so is pushing to get the issue of a referendum debated at next month's Labour conference. This is not guaranteed as the conference as a whole would need to decide that this would be a priority for debate.
Even if it clears this hurdle, some left-wing members of the party - who usually back Jeremy Corbyn - would need to offer their support, for any motion on Brexit to be successful,
With this need to appeal to Corbyn supporters in mind, the internal memo to members of the People's Vote campaign does explicitly state that any change in Labour policy to support a new referendum should not be used to undermine or damage the party leadership.
Many of the more prominent Labour politicians associated with the campaign have been critics of Jeremy Corbyn.
So the memo - drafted by Tom Baldwin, a former adviser to Ed Miliband when he was Labour leader - urges supporters to make clear that the People's vote should not be seen as a "nascent" centre party, a fear of some on the left.
It emphasises that "whatever views supporters have about the overall direction of the Labour Party, the campaign should not be used to attack Jeremy Corbyn personally" and that the tone the campaign takes will be crucial to its success.
With the left-wing group Momentum about to consult its members on Brexit, the People's Vote is hopeful that Labour's policy could change before Parliament votes on the final deal.