Chancellor Angela Merkel is facing a possible revolt by a contingent of conservative MPs over her open-doors refugee policy as the New Year’s Eve sexual assaults in Cologne threaten to shake the country’s politics.

Sceptics in the chancellor’s ruling CDU/CSU bloc are exploring a formal call for a full border controls and the barring of almost all refugees arriving at Germany’s frontiers.

While it is by no means clear that they will win wider support, their push highlights the growing unease in the CDU and its Bavaria-based sister party, the CSU, where many had already questioned Ms Merkel’s response to a historic influx of migrants.

The New Year’s assaults, in which hundreds of women claim to have been sexually assaulted by men of north African or Arab appearance, has crystallised the public’s fears about the consequences for Germany of accepting so many newcomers.

Ms Merkel’s challengers are arguing that the frontiers should be secured by enforcing EU rules — the so-called Dublin system — under which asylum seekers are required to file claims in the first EU country they reach. Germany would therefore be entitled to turn away arrivals at its land borders, including the Austrian frontier, where almost all refugees arrive.

Such a radical shift would require the deployment of huge contingents of border guards and potentially lead to the collapse of the Schengen free-travel zone at the heart of the EU.

But, with 1.1m refugees having arrived in Germany last year — and more expected in 2016 — Ms Merkel’s critics argue that they have no option. They say time is running out for her hopes of securing international agreements with Turkey and EU partners to both reduce flows at the EU’s external borders and more evenly distribute those asylum seekers accepted into the EU.

The sceptics are pondering whether to put a motion before the next parliamentary bloc meeting on January 26 proposing that “the German national borders must be reliably secured”.

The sceptics today have far less support than would be needed to force the chancellor’s hand: Bild newspaper reported the motion has been signed by 40 MPs out of the bloc’s 311. Ms Merkel last year comfortably survived a rebellion by 60 CDU/CSU MPs who voted against the government in parliament over the latest Greece loan package.

Volker Kauder, the CDU/CSU chief whip, said: “It is true that there are differing opinion over the question of how we achieve the goal of reducing the number of refugees.”

Christian von Stetten, a CDU conservative organising the motion, has declined to confirm the Bild figures. However, he said the parliamentary group had to build on the decisions taken in the recent CSU parliamentary group meeting in Bavaria, where CSU MPs backed securing the borders.

The CSU has long taken a tougher view on immigration than the CDU, with its leader, Horst Seehofer, demanding a 200,000 annual limit on refugees, defying the chancellor’s no-limit policy.

With 56 MPs, the CSU has a significant but far from decisive voice in the combined bloc. One CDU MP present at this week’s parliamentary group meeting said it was hard to estimate now how many MPs were ready to confront the chancellor’s policy.

Few would ditch Ms Merkel: they want a new policy, not a new leader. It is common for motions to be circulated and later dropped.

Meanwhile, in Wednesday’s parliamentary debate, the government won cross-party backing for plans to strengthen the criminal code so that immigrants convicted of serious crimes, including sex attacks, can be deported more often than now.

Separately, CDU politicians called for the resignation of Ralf Jäger, the Social Democrat interior minster for North Rhine-Westphalia, which includes Cologne. They want Mr Jäger, who has removed the Cologne police chief, to take responsibility for police failures on New Year’s Eve.

The tally of women alleging assault or robbery has risen to 560