Dassanayake, also a former Navy intelligence chief, is under investigation over the enforced disappearance of 11 youths of Colombo during the previous regime.
A few months ago, courts issued a foreign travel ban on him.
Moving a motion today (06), his lawyer requested that his client be allowed to go overseas for a one and a half year training.
Noting the police have completed investigations against him and the report sent to the attorney general, the court rejected the request.
The court also ordered the CID to submit a report on September 27, BBC Sandeshaya reports.