BERLIN—Political backlash against German chancellor Angela Merkel intensified on Friday as her most conservative ally threatened to take the government to court if it doesn't adopt measures to limit the flow of migrants into Germany.

As the number of migrants flocking to Germany has continued to swell, Ms. Merkel's open-arms policy has exposed a widening rift inside the government over how to tackle the biggest migrant tide Germany has witnessed since WWII.

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