Offenders serving "part-time" prison terms will be allowed to vote in the general election if they are not in jail on 5 May, the BBC has learned.
Prisoners are currently banned from voting in local and general elections
The decision affects about 40 whose sentences are split between jail and the community.
The development comes as a government appeal against a European ruling that prisoners have voting rights has begun.
Officials say a general ban on voting is a legitimate part of punishment.
Last year prisoner John Hirst, 53, won a challenge at the European Court of Human Rights that current rules denying prisoners a vote breached their rights.
A final verdict will be delivered later in the year.
The government argues that people who commit crimes serious enough to warrant a custodial sentence should forfeit the right to have a say in how their country is governed.
But the Department of Constitutional Affairs has disclosed that an exception will be made for those on the intermittent custody scheme, where offenders spend part of the week in jail and the rest of the time in the community.
BBC correspondent Danny Shaw said that with only 40 prisoners involved in the scheme, the numbers voting were unlikely to be large enough to affect even the most marginal of seats.
But the move had set a precedent, he added.
Juliet Lyons, director of the Prison Reform Trust, said the decision "opens the door to governments saying prison doesn't have to be about civic death".
"Instead, prison is about reform, rehabilitation, about becoming a citizen and staying a citizen," she told BBC Radio 4's Today programme.
"In most council of European countries, prisoners simply can vote."
It was "excellent" to see the government taking a decision itself rather than being "pushed into it" by Europe, she added.
On Tuesday, government lawyers began their appeal in the Hirst case before a 17-judge grand chamber of the European Court of Human Rights, in Strasbourg.
Hirst is currently serving a life sentence for manslaughter at Rye Hill prison, Warwickshire.
In 2001, he lost his claim for voting rights in the High Court in London.
But last April, a European Court of seven judges agreed with him and ruled that his human rights had been breached by the government.
He was awarded £8,000 in costs and expenses.
Former Conservative prisons minister Anne Widdecombe said that affording prisoners the right to vote would do nothing to stop most prisons being "dens of idlement".
Reforming prisoners was not about "giving them a say in how the rest of us are governed", she told BBC Radio 4's Today programme.
"You should worry about things like literacy, numeracy, qualifications and the structure of an ordinary working day."
That was the "overwhelming importance of our prison system", she added.