Wanadoo and BT have both been hit by the blockade, according to sources familiar with the situation, and early reports have claimed that Telefonica and China Telecom are also hit.
The move has caused frustration among expatriates worldwide. The site, www.fvap.gov, is the online home of the Federal Voting Assistance Program, which allows registered American voters to vote in their absence.
The US Department of Defense, which runs the Federal Voting Assistance Program, confirmed on Tuesday that "certain Internet service providers" were being blocked from accessing www.fvap.gov.
"The goal is to make it more difficult for hackers to deface and/or hijack the website," said Lieutenant Colonel Ellen Krenke.
"The Federal Voting Assistance Program is currently working on a solution to this problem and hopes to have it in place as soon as possible."
Lieutenant Colonel Krenke added that she didn't have a list of the ISPs affected.
BT is still investigating whether it has been hit by the blockade, but sources within the company have reported that they are unable to access www.fvap.gov.
It's unclear at this stage how the blocking is taking place. The US Department of Defense may have identified the IP blocks used by major ISPs, or it may be relying on the Autonomous System Number that is used to identify an Internet service provider's network as a whole to other ISPs.
A spokeswoman for Democrats Abroad said she was aware that customers of BT and Wanadoo had been affected, and was unimpressed by the claim that the measure would prevent hacking.
"I can't see the advantage of hacking the site," she said, explaining that voters still have to order, receive, sign and return their voting forms. "You've not going to change the voting [just by hacking the site]."
The deadline for submitting absentee ballots in some states is 2 October, so there is concern that the blockage could prevent some people from taking part in the election.
Democrats Abroad have responded to the Department of Defense blockade by setting up a separate site, called www.overseasvote2004.com, to handle absentee ballot requests.
According to the Department of Defense there are other ways of taking part in the election from overseas.
"In the meantime, overseas voters can contact their embassy or consulate, use the FVAP toll-free number or contact their local election official or secretary of state via telephone or the internet for more information on obtaining an absentee ballot," said Lieutenant Colonel Krenke.
ZDNet UK's Jonathan Bennett contributed to this report.