Ed Miliband has called for search and rescue operations for migrants crossing the Mediterranean to be "immediately restarted".
Speaking at a Scottish Trade Union Congress event in Ayr, the Labour leader said it was a mistake to end the search and rescue operations and that he would restart them if he was elected prime minister in next month's general election.
Hundreds of migrants attempting to travel from Africa to Europe are feared dead after a boat sank over the weekend.
"I believe all of us have been shocked and appalled by events in the Mediterranean," said Miliband. "We cannot stand by as people drown in European waters simply to escape conflict in Libya and Syria.
"It was wrong to end the search and rescue operations and they should be immediately restarted. We must recognise our humanitarian responsibility. If I was prime minister, I would be working with other European leaders to act."
He added: "Our country is known throughout the world for our generosity of spirit. It's time to reflect that and take action in the face of this terrible tragedy."
Meanwhile, Downing Street confirmed that David Cameron had discussed the issue with the prime ministers of Italy and Malta.
"The prime minister agreed that the EU needed to respond urgently to address the crisis to try to prevent further deaths," a spokesperson said. "All three leaders agreed that the criminal networks behind human traffickers were primarily to blame for this tragedy and that the highest priority had to be action to disrupt their activities."
However, Cameron did not immediately call for a return to search and rescue operations, instead calling for a "comprehensive approach to the crisis", including action to stabilise the Libyan government, secure migrants' home countries, and undermine the work of human traffickers.