An army charity claims it is nearly impossible to get ballot papers to those in conflict zones
Thousands of British troops in Afgfanistan have been "disenfranchised" in the election according to the Liberal Democrat candidate for Wrexham.
Tom Rippeth said many have not been registered to vote and there are problems getting postal votes back to the UK in time to be counted for 6 May.
The Army Families Federation charity claims it is nearly impossible to get ballot papers to conflict zones.
Some 65% of the 9,500 service personnel in Afghanistan are registered to vote.
The Ministry of Defence told BBC Wales that information about voting is issued to units overseas, and soldiers are advised to vote by proxy through a loved one in the UK.
Electoral laws mean postal votes could not be issued until 11 days before the election, making it difficult to ensure that they get out to Afghan conflict zones and back to the UK in time.
Mr Rippeth said: "We now hear that their voices won't count in the forthcoming General Election. That is nothing short of a disgrace."
He said that the Canadians sent out ballot boxes with a sufficient time gap to ensure that the votes counted and that US troops will vote on the internet during the mid-term election later this year.
Labour candidate Ian Lucas said the independent Electoral Commission has worked hard alongside the Ministry of Defence to explain the best ways for armed forces personnel to vote.
Mr Lucas said: "This explains how to register as a service voter, how to obtain a proxy vote, rather than a postal vote and other information.
"The campaign sends out the information every autumn - in good time before any election.
"I have attended many armed forces events in Wrexham and have had no-one raise concerns about their vote with me."
Conservative candidate Gareth Hughes said: "Now our fighting men and women are to be deprived of the most fundamental right in a free society, which is to exercise the right to hire and fire their political leaders.
"Sadly, whilst they are fighting to ensure amongst many things the right for the Afghan people to vote in free elections they are losing that right for an election in Britain."
Plaid Cymru candidate Arfon Jones called the war in Afghanistan "the forgotten scandal" of the election campaign and argued that "the establishment parties don't want to discuss the matter because they have backed the war".
"I hope the presence of more Plaid MPs in Parliament will send a clear message that we want the troops home safely and to ensure those who have injuries or psychological damage receive the best possible care."
UKIP candidate John Humberstone said: "In this day of emails and online machinery for all sorts of things surely it's not beyond the wit of man to set up a system for them to vote.
"I am not going to enter into party mud-slinging, but I do feel that when we expose people to such dangerous conditions, as in Helmand Provence, to deny them a right to a voice in deciding who governs them is very, very, wrong indeed."
Julie McCarthy, chief executive of the Army Families Federation, earlier told the BBC there have been problems with the proxy vote system in the past.
She said: "When we were in Germany, my father was my proxy vote and if he voted the way I wanted him to I'd be very surprised.
"And I know of a few people who have that problem and not everybody has somebody that they can trust, that they want to give that proxy vote too.
"Also, postal voting really for me just doesn't work at all because of that 11 day window.
"Our Germany branch did some trials in the last couple of weeks and the time taken to get papers from here in Wiltshire over to Germany, signed as you would a ballot paper, and back, was between eight and 12 days."
A Ministry of Defence spokesman told BBC Wales the ministry was working with the Ministry of Justice and the Electoral Commission to see what could be done for people serving in Afghanistan.
He said: "No plan is going to be infallible if we're talking about Afghanistan."
See a list of the candidates standing in the Wrexham constituency