Seizing on slipping poll numbers and mounting scepticism among Germans about the country's ability to handle the influx, which brought nearly 1.1 million newcomers last year, the Christian Democratic Union (CDU) lawmakers said Merkel must face up to reality.
"In light of the developments in recent months, we can no longer speak of a great challenge -- we are on the verge of our country being overwhelmed," they wrote, in the letter addressed "Dear Madam Chancellor" and obtained by AFP.
"We do not want to divide the CDU parliamentary group -- we are only asking for the law to be applied," one of the initiators of the letter, Christian von Stetten, told AFP.
The deputies said that quick processing of asylum applications, housing in "appropriate" conditions and "successful" integration of newcomers were all impossible if "the number of arrivals remains this high or begins to climb again in the spring".
Merkel, who has attempted to rally Germans with the slogan "We can do it", has pledged a "tangible reduction" in the number of asylum seekers arriving in Germany in the coming months.
This would entail a raft of new measures at the national level and in cooperation with EU partners and neighbours of war-ravaged Syria, from which around 40 percent of last year's new arrivals came.