Seán Mitchell (19) is too young to run for Stormont in March, even though he could stand for the Welsh Assembly in Mr Hain's own constituency eight weeks later.

His lawyers plan to challenge the NIO decision that deliberately prevents people between 18 and 21 from running in Northern Ireland when they will be permitted to run for office in the other region run by Mr Hain - Wales - and the rest of the UK in May.

"The Government believes people over the age of 18 can stand elsewhere in the UK," said Mr Mitchell, who wants to run on an anti-water charges ticket.

"Why not give those rights to people here in the North?

"Given all the talk about Asbos and anti-social behaviour, isn't it strange that the Government is denying young people the chance to engage in the political process?"

Last month the Government triggered new legislation that drops a previous age restriction for candidacy and allows under-21-year-olds to run for office - but only in the rest of the UK.

That means they can stand in May for the Scottish Parliament elections, the Welsh Assembly poll, council elections in England, Wales and Scotland, and London's mayoral race. They will also be eligible for future Westminster elections.

But the commencement order for the new legislation specifically excluded Northern Ireland, even though the Government had already scheduled it.

Other changes in election law were also kept back.

The NIO says the changes will be implemented later this year - after the Assembly election - but officials haven't explained why the new measures were held back.

Mr Mitchell said he wants to run for office to promote a campaign against paying water charges and highlight other issues, including policing and the increased deployment of MI5 in Northern Ireland.

"I think it is an absolute disgrace," he said. "This means people in Northern Ireland are disenfranchised in this election."

Mr Mitchell's lawyers have written to the Government asking for a change in the law immediately so he and other under 21-year-olds can stand in the March Assembly contest.

They say they will file papers for an emergency judicial review at the end of this week if the Government hasn't agreed. That would make it the latest in a series of Mr Hain's decisions that have been dragged before the courts.

"It's not acceptable that young people in Northern Ireland do not enjoy the same rights as people elsewhere in the jurisdiction," said Barbara Muldoon, Mr Mitchell's legal representative.

The Department of Constitutional Affairs triggered the Electoral Administration Act, but indicated yesterday that it was the NIO's decision to hold back the legislation from Northern Ireland.

A Northern Ireland Office spokesman did not offer a reason for preventing young people from standing in the March Assembly election.

"It is the Government's intention to bring this provision, along with other relevant provisions of the Electoral Administration Act 2006, into effect in Northern Ireland later in the year," the spokesman said.