Shalom's move means that eight Likud ministers support the plan for a withdrawal from the Gaza Strip and the northern West Bank, and only five oppose it.
The Likud's 200,000 members will vote on the issue on May 2 but the ministers' decisions are bound to influence them.
Shalom said he was swayed during a meeting with Sharon Monday, in which they discussed the plan, the American quid-pro-quo for the pullback, and how to implement the plan.
Shalom especially noted the U.S. administration's readiness to help implementation. He noted President Bush's statement that Palestinian refugees should return to the Palestinian state rather than to Israel, and that the final status agreement with the Palestinians should consider the settlement blocks Israel has built in the West Bank.