The attack started in late October but has grown in the last few days to overwhelm the nation's link to the net, said security firm Arbor Networks.
Reports from Burma say the disruption is ongoing.
The attack, which is believed to have started on 25 October, comes ahead of closely-watched national elections on 7 November.
International observers and foreign journalists are not being allowed into the country to cover the polls - which many Western leaders have said will not be free or fair.
It will raise suspicions that Burma's military authorities could be trying to restrict the flow of information over the election period.
BBC News
