David Cameron has issued a fresh warning that the migration crisis could push the UK into voting to leave the European Union as he continued talks aimed at striking a new deal with Brussels.
The Prime Minister said the short-term impact of the influx of migrants arriving in the EU could be a desire among voters to "push Europe away".
But in a hint that he he may be considering a longer campaign before his promised referendum on EU membership, the Prime Minister said that the "longer term reaction" to the twin migration and eurozone crises could be to make sure that there was a relationship with Brussels that "works".
His remarks come as the number of migrants claiming asylum in the EU this year officially passed one million according to Eurostat, the EU's statistics agency.
The Prime Minister was in Poland on the latest leg of his diplomatic offensive to win backing for the reform programme he hopes will allow him to recommend the UK remains within the 28-member bloc.
A major obstacle is his call for a ban on EU migrants claiming in-work benefits in the UK until they have been in the country for four years, which he hopes will act as a disincentive for citizens of member states seeking to come to Britain.
But in an interview with the Spectator, Mr Cameron said the wider migration crisis triggered by people seeking to come to Europe could harden British opinion against the EU.
In the interview, reported in the Daily Telegraph, Mr Cameron said: "I think with both the eurozone crisis and the migration crisis, the short term impact is for people to think, 'oh Christ, push Europe away from me, it's bringing me problems'."
In an indication that he may consider delaying a referendum until nearer the end of 2017 deadline he has set, Mr Cameron added: "I think the longer term reaction might actually be, well if they are going to have a single currency and they are on our doorstep and they are going to try and make it work, let's make sure our relationship with them works and then we have safeguards, not least for our vital financial services industry so that the system doesn't work against us."
The Prime Minister, who will continue discussions with his Polish counterpart Beata Szydlo today, added: "The short term reaction can be get me out of here, the longer term reaction is we must find a better way of working with our partners because we share the same challenges."
Mr Cameron has already been forced to concede that there will be no agreement on his reform agenda at this month's summit of European leaders, with a showdown on his demands now likely in February.
In a further indication of the challenges he faces, European Council president Donald Tusk reportedly claimed that no other EU country fully backed his reforms.
The Daily Telegraph reported that Mr Tusk told a meeting of MEPs that there was "no consensus", adding: "In fact, 27 member states are very sceptical. Indeed, they are against."
The newspaper reported that Mr Tusk warned that Mr Cameron "will not be satisfied 100%" with the outcome of negotiations.
After talks in Bucharest with Romanian president Klaus Iohannis on Tuesday, Mr Cameron stressed the importance of his measures aimed at curbing the number of EU citizens heading to the UK.
He said net migration to the UK was "not sustainable" and stressed EU members should be allowed to reform benefits rules to reduce incentives.
He acknowledged that "some areas are more difficult than others, particularly the reforms I have proposed on welfare".
Eastern European countries including Romania and Poland have been major sources of migrant workers coming to the UK, and their politicians are sensitive to any measures which could discriminate against their citizens.
But Mr Cameron insisted: "Net migration in the UK is running at well over 300,000 a year and that is not sustainable.
"So we do need to find ways to allow Member States to make changes to their social security systems that will help them to deal with this issue."
During his visit to Warsaw, Mr Cameron will also hold talks with Polish president Andrzej Duda.