The Department of Homeland Security will begin a review on Thursday of all deportation cases before the immigration courts and start a nationwide training program for enforcement agents and prosecuting lawyers, with the goal of speeding deportations of convicted criminals and halting those of many illegal immigrants with no criminal record.

The accelerated triage of the court docket — about 300,000 cases — is intended to allow severely overburdened immigration judges to focus on deporting foreigners who committed serious crimes or pose national security risks, Homeland Security officials said. Taken together, the review and the training, which will instruct immigration agents on closing deportations that fall outside the department’s priorities, are designed to bring sweeping changes to the immigration courts and to enforcement strategies of field agents nationwide.

According to a document obtained by The New York Times, Homeland Security officials will issue guidelines on Thursday to begin the training program and the first stages of the court caseload review. Both are efforts to put into practice a policy senior officials had announced in June, to encourage immigration agents to use prosecutorial discretion when deciding whether to pursue a deportation.

The policy, described in a June 17 memorandum by John Morton, the director of Immigration and Customs Enforcement, suggested that the Obama administration would scale back deportations of illegal immigrants who were young students, military service members, elderly people or close family of American citizens, among others. While the announcement raised excited expectations in Latino and other immigrant communities, until now the policy has been applied spottily, deepening disillusionment with President Obama in those communities.