EDINBURGH East MP Sheila Gilmore is asking Labour voters in her constituency to decide which of the party's leadership candidates she should vote for.
She is sending out ballot papers to around 1,000 identified Labour supporters and is inviting others to have their say via her website.
Ms Gilmore said she had decided on the US primary-style election so the choice of Labour's new leader could be made by the widest possible range of people.
She said: "We talk a lot about getting more people involved, but we don't always get round to doing it. This is an opportunity to engage people in what will be a very important decision and give them a real say."
Five contenders are battling it out to succeed Gordon Brown as Labour leader - David and Ed Miliband, Ed Balls, Andy Burnham and Diane Abbot.
Ms Gilmore nominated Diane Abbot to stand in the leadership election in a bid to ensure as wide a choice as possible, but made clear she was not committed to voting for her.
She said she was "genuinely not sure" who she wanted to win, but would cast her vote according to the result of the "primary" in Edinburgh East.
The local Labour party is now writing to Labour voters across the constituency with a ballot paper to be returned to a secure freepost address.
However, Ms Gilmore won over 17,000 votes at the general election in May, so in order to reach a bigger proportion of those who voted for her, she is also offering people the opportunity to vote online.
By visiting www.edinburgheast.net and going to the Leadership elections page, constituents can have their say as long as they provide their name and address so their details can be verified against the electoral roll. They will also be asked to confirm they voted Labour in the election.
Ms Gilmore has invited the five candidates to give short statements she can post on the website.
The new Labour leader will be elected by an electoral college made up of three different groups, each with one-third of the votes - MPs and MEPs; Labour Party members; and trade unions and affiliated organisations. Ms Gilmore said that meant her vote as an MP was worth the same as thousands of Labour members and tens of thousands of trade union members.
MPs will vote by ranking the candidates in order of preference. But Ms Gilmore is asking constituents to place a cross against just one of the hopefuls. She will then draw up her preferred order based on how many votes each candidate gets.