HMS Bulwark, which has rescued more than 2,900 migrants trying to cross the Mediterranean over the past six weeks, is to be withdrawn from the role, George Osborne has confirmed.

The chancellor insisted Britain will continue to play “our full part” in search-and-rescue operations in the Mediterranean, but what exactly will replace the 19,000-tonne warship, which is capable of carrying two helicopters, is unclear.

Defence sources confirmed HMS Bulwark’s temporary deployment was only for an initial 60 days, which will expire in three weeks. Downing Street said there was no definitive date yet for the withdrawal of the former Royal Navy flagship. The Ministry of Defence has said it is “planning for a seamless transition of assets”.

Asked by the Guardian if a replacement for HMS Bulwark was guaranteed, a Downing Street spokeswoman said: “The prime minister and the chancellor have talked about the UK having a moral role to play to help save lives. I’m sure in the discussions that are continuing this week and next ahead of the June European council, they will be looking at the options.”

The Italian government has warned that it will refuse landing rights to migrants and asylum seekers rescued by foreign ships if there is no EU agreement on a responsibility sharing plan for 40,000 of the migrants who are expected to arrive in Italy and Greece over the next two years.

This could make things very difficult for any vessel deployed to replace HMS Bulwark, as it was sent only on the condition that those rescued would be taken to the nearest port and would not have recourse to claiming asylum in Britain.

Osborne, standing in for David Cameron at prime minister’s questions, had been challenged by Labour’s Hilary Benn over the Guardian report on Tuesday that the future of HMS Bulwark’s deployment was under active review. He asked: “Having made a grave error last October in withdrawing support from the Mare Nostrum search-and-rescue operations, can you assure the house that the government will continue to save the lives of those in peril at sea?”

The chancellor said he could assure him that Britain would play its full part in search-and-rescue operations in the Mediterranean: He said: “As I understand, there is essential maintenance that needs to be carried out on HMS Bulwark, which is clearly an operational issue.

“But no one should in any way doubt Britain’s determination to play its role in helping with this situation.

“Taking people out of the water and rescuing them and their lives of course is absolutely essential. We are a humanitarian nation and we need to deal with that.

But in the end you have to break the link that enables someone to get on a boat and then claim asylum in Europe and spend the rest of their lives on the European continent.”