Britain will "strongly" oppose scrapping the EU rule that means refugees must claim asylum in the first country they enter, a Cabinet minister has said.
David Cameron is braced for a battle after the European Commission signalled it will propose dropping the principle as part of reforms to the so-called Dublin regulation.
The move could leave the UK and other northern European countries more open to migration flows, as the majority of refugees from Syria have been arriving in the south and trying to continue their journey overland.
It is a fresh headache for the Prime Minister as he renews his push to finalise new British membership terms, making a keynote speech and holding talks with EU counterparts at the World Economic Forum in Davos before visiting Prague on Friday.
Mr Cameron still hopes to secure a deal for a Brussels summit on February 18 - but officials admit it is an ambitious timescale. Further delay would make it impossible to hold the in-out referendum in June.
The Commission expects to table a formal proposal for replacing the Dublin rules in March. States such as Greece and Italy have been demanding reform, complaining that they are bearing the brunt of the refugee exodus from Syria.
Although the UK has an opt-out from wider EU asylum policy, it has signed up to the Dublin principle.
At a briefing for journalists, International Development Secretary Justine Greening stressed that negotiations were still under way and reiterated that the Syrian crisis needed to be addressed at source.
But she said the UK had been able to use the rule to deport asylum seekers back to the EU country where they first arrived.
Asked whether the Government would be concerned at any effort to change the principle, Ms Greening said: "On Dublin, yes, we would be concerned and strongly against any change from that initial country status that we have got right now. It is important."
A European Commission spokesman said: "As announced last September, the Commission will set out a reform of the Dublin System, with proposals due by March.
"The revision will aim at ensuring a fairer distribution system amongst member states and to ensure that Dublin becomes a solidarity sharing system."
Commission president Jean-Claude Juncker has insisted that changing the Dublin rules is a "necessity" because "the existing system clearly does not work properly in all respects".
Matthew Elliott, chief executive of Vote Leave, said: "Our membership of the EU means that we've given up control of the UK's asylum system to the European Court of Justice.
"The EU is incapable of dealing with the global challenges it faces as the only solution it proposes to any issue is to seize more power for Brussels."