David Cameron has been told to take responsibility for the thousands of refugees dying in the Mediterranean, as the issue overshadowed the election campaign.
Campaigners say the UK is particularly responsible for the deaths after it cut back on funding for search-and-rescue operations in a bid to discourage others coming over from Africa.
"Britain should immediately pledge funding to help with the search and rescue operation in the Mediterranean," Green party leader Natalie Bennett said.
"We cannot simply stand by while hundreds of people drown in our continent's seas. The charities saving people’s lives in the Mediterranean must be properly resourced, and Britain must help with this.
"In the longer term we must push the EU into rebuilding a scheme to rescue migrants in the Mediterranean – we shouldn’t have to rely on the heroic work of charities to undertake this basic humanitarian task.
"It's easy to forget that, behind all of the hard talk on migrants, are people who are desperately fleeing wars, hunger and disease."
The attack comes as up to 900 migrants are thought to have died in the latest tragedy.
The tiny vessel is thought to have capsized south of the Italian island of Lampedusa. So far, only 28 people have been rescued.
At least another 900 migrants have died crossing the Mediterranean this year, although the real figure is likely to be far higher.
The current tragedy comes just days after 400 were thought to have died when their boat capsized while making the same journey.
Last October, Foreign Office ministers quietly slipped out the news that the UK had cut its search-and-rescue funding in a bid to discourage further refugees, most of whom are fleeing Libya.
Foreign Office minister Lady Anelay said: "We do not support planned search and rescue operations in the Mediterranean," adding it provided an "unintended 'pull factor', encouraging more migrants to attempt the dangerous sea crossing and thereby leading to more tragic and unnecessary deaths".
She added: "The government believes the most effective way to prevent refugees and migrants attempting this dangerous crossing is to focus our attention on countries of origin and transit, as well as taking steps to fight the people smugglers who wilfully put lives at risk by packing migrants into unseaworthy boats"
The funding decision came at the end of Italy's sea operation, Mare Nostrum, which was not restarted. The EU has failed to set up anything to replace it with, leading many refugee and human rights groups to accuse it of direct responsibility for this year's deaths.