The armed forces are set to take charge of operations looking to limit migrant crossings in the English Channel.
Home Secretary Priti Patel said the MoD had been commissioned as a "crucial operational partner to protect our Channel against illegal migration".
But Defence Select Committee chairman Tobias Ellwood criticised the plans as "rushed" and a "massive distraction" for the military.
The move could begin within weeks, the BBC understands.
Rear Admiral Mike Utley, the commander of the UK strike force, has been appointed to co-ordinate the plans.
The number of people who crossed last year was treble that of 2020. On Saturday alone, 197 people were intercepted or rescued attempting to cross the English Channel, the Home Office said. French authorities stopped 95 people on five vessels from making the journey.
Ministry of Defence (MoD) sources told the BBC discussions about the armed forces working with the Home Office and UK Border Force had been taking place for several weeks.
They said no decisions had been made over how the Royal Navy or other services would be involved, and there was no indication it would involve pushing migrant boats back to France.
That is despite Ms Patel suggesting in the House of Commons that the armed forces could be involved in the controversial policy, saying this was "absolutely the policy of this government".
A source added the details of how the military could co-ordinate operations were still to be worked out, while another said there was "trepidation" within the MoD about getting involved in such a complex issue.
It is understood overall policy on migration and migrant management would still be led by the Home Office.
But the plans, first reported in the Times, could see Prime Minister Boris Johnson give the navy authority over government vessels in the channel.
Figures compiled by the BBC show at least 28,431 migrants made the journey in small boats in 2021 - up from 8,417 people the year before - despite huge UK investment in France to prevent crossings.
On 24 November at least 27 people died after their boat sank, in the largest single loss of life in the Channel since records began in 2014.
Ms Patel told MPs crossings were "unfair, unacceptable and lethally dangerous" as well as "totally unnecessary". She said France, and other EU member states, were "safe countries with long-established asylum systems".
She added she had asked for "naval MoD assets and support back in 2020". "No one department can resolve the complex issue of Channel crossings on their own," she added.
Asked about the move on BBC Radio 4's Today programme, Education Secretary Nadhim Zahawi said it would help to avoid a situation where "Border Force or other vessels in the Channel are being co-ordinated separately".
But Mr Ellwood, who is also a former veterans minister, described the proposals as being "rushed out".
Speaking to Sky News, the Conservative MP said the plans were a "massive distraction" for the military amid growing threats from Russia and China, adding: "This isn't what our navy should be doing."
For a prime minister who promised his Brexit deal would see the UK take back control of its borders, the failure to stop record numbers of asylum seekers arriving on the Kent coast in small boats is a political embarrassment.
However, putting a rear admiral in charge of operations in the Channel is unlikely to make much difference.
The Royal Navy may be able to provide better intelligence on where inflatables and dinghies are expected to make landfall, but the great majority of those coming to seek sanctuary in Britain are already intercepted before they reach the coast.
Plans to push small boats back into French waters appear to have been deemed too dangerous and the often-trailed idea of processing migrants "offshore" in another country has so far failed to materialise. Migrant arrival facilities in Kent are currently being expanded.
The reason for the increase in asylum seekers risking the perilous journey across the Channel is that Covid and improved security at Calais has effectively closed traditional points of entry.
The government and refugee organisations agree the long-term solution is the creation of official safe routes for migrants fleeing conflict zones.
Shadow home secretary Yvette Cooper said the government had failed to do the serious and practical work with France needed to stop lives being lost and criminal gangs profiting from crossings.
The Labour MP said the government had brought the navy in to patrol the Channel in 2019 but the two vessels used had intercepted no boats and had cost the government £780,000.
"They need to explain what is different in these latest plans," she said.
A government spokesperson said the public had had enough of people smugglers disregarding immigration laws and ministers were bringing in long-term changes.
A Ministry of Defence spokesman said unacceptable numbers continued to cross and the government was exploring every avenue to stop them.
Responding to the plans, CEO of the Refugee Council Enver Solomon described them as "cruel and inhumane".
"It's a desperate move by a government that isn't able to find solutions that will ensure an orderly, manageable and fair asylum system," he added.
But Dover MP Natalie Elphicke said the news sent "a clear message" over how serious the government was about putting a stop to small boat crossings which were incredibly unsafe.
In March last year, Ms Patel set out plans to overhaul the way people applying for asylum in the UK are treated.
Under the proposals, people who arrive in the UK by what the government calls illegal means to claim asylum will no longer have the same entitlements as those who arrive through proper channels.
The Labour Party has criticised the plans for lacking compassion and being ineffective at discouraging crossings. Human rights lawyers warned they would be unlawful because they ignored Britain's international obligations.
In December, four Iranian men who crossed in small boats had their convictions for immigration offences quashed by the Court of Appeal, which concluded it had not been proven they intended to enter the UK illegally.