British politicians have called for a "clear end point" to UK military tackling migrant crossings in the English Channel, with command of the operation to be handed back to the Border Force.
The House of Commons defence committee criticised the Home Office and Ministry of Defence for their "public disagreement" over the use of tactics to push small boats trying to reach England out of British territory.
It called exchanges in the Commons and on Twitter "deeply unedifying".
UK Prime Minister Boris Johnson approved plans in January for the military to take over command of the initiative, known as Operation Isotrope, from the Border Force.
Defence Minister James Heappey said in February that the operation would seek to prevent the arrival of small boats "on their own terms" in the UK while protecting lives at sea.
Mr Heappey said the ministry was working closely with the Home Office and others.
But a report from the defence committee has called Isotrope an "ill-defined policy, prematurely announced", saying that "the best-case scenario for the Royal Navy is that it will leave with its reputation unharmed".
Labour MP John Spellar, the committee's vice chairman, said the armed forces "cannot become a fourth blue light service".
"As we've made clear in recent reports, the Ministry of Defence, and especially the Royal Navy, is already over-stretched and under-resourced," Mr Spellar said.
"An additional responsibility, particularly one as substantial as intercepting Channel migrant crossings, places huge pressure on a department with a demanding-enough brief.
"In order to meet the demands of this new policy, we should be under no illusion that sacrifices to defence will be made elsewhere.
"The government should set clear parameters for Operation Isotrope and announce a definitive end date and handover back to the Border Force."
He criticised the Home Office and ministry for "squabbling" over how to manage the operation, which he said "severely undermines public confidence in government".
Home office and ministry at loggerheads
The defence committee report said Mr Heappey repeatedly told the Commons that the Royal Navy and Royal Marines would not engage in "pushback tactics".
"I have been clear that the Royal Navy and the Royal Marines will not use the tactic, principally because they simply do not have the platforms that are appropriate for doing so," he said on January 18.
"Arguably, the Border Force does. It has been doing trials with it, so it remains an option.
"But let us be clear: there are parts of the Channel in which it definitely cannot be done, and there is a small part of the Channel in which it might possibly be done. That is for the Navy commander to consider in due course."
However, two weeks later, Home Secretary Priti Patel told the home affairs committee that Mr Heappey had given his "view" but that the operational work between the two departments was "in progress".
"It is wrong to say anything specific with regards to work operationally that is still being planned. That work has not been completed yet," Ms Patel said.
This was followed up by a tweet from the ministry, which said: "The Royal Navy and the Royal Marines will not be using pushback tactics in the English Channel, although a military commander will retain the existing ability to instruct Border Force to use them when appropriate. A further update will follow in due course."
The defence committee report said: "A public disagreement between two great offices of state, played out in the House and on Twitter, is deeply unedifying.
"If one of the objectives is to restore public confidence, then this disagreement has done the opposite and the operation has failed."
It accused the government of announcing the policy "before finalising the details", which it said was "bad practice, particularly when that policy area is so sensitive".
The report also claimed ministers had provided the "bare minimum of information" around "the ways and means which will be used to achieve the strategic objective [let alone the strategic objective itself]".
"This is compounded by the fact that, more than six weeks after the decision had been announced and we had requested more information from them, we have not received any evidence, written or oral, from either of the departments involved," it said.
The committee said it was "seriously concerned by the lack of a clear end point for this operation".
"The [ministry] has its own policy failings which it needs to remedy. It should not be made responsible for the failings of other government departments as well."
It called on the Home Office and the ministry to agree on "realistic indicators of success" and explain them to both Houses of Parliament.
"Furthermore, we recommend that an agreed date for the operation to be handed over to the Border Force is published, with the option of extension if so required," the report said.
"This cannot be an open-ended deployment, occupying scarce Royal Naval vessels and personnel."
Mr Spellar said: "Calling in the Navy is not a quick fix to a sticky issue. The government should prioritise outcomes over optics."
A government representative said: "We are determined to break the business model of dangerous criminal people smugglers and prevent further loss of life in the Channel.
"We have been exploring every avenue to prevent further crossings and the Home Office and Ministry of Defence continue to collaborate on a more visible role for the armed forces in the Channel.
"None of the Royal Navy ships earmarked for deployment in the Channel have any role, or even possible role, in the UK's response to Russian aggression in the Ukraine.
"It is right that we pursue all options to prevent illegal crossings, secure our border and protect life at sea."