She also appointed a former royal marine as her “clandestine Channel threat commander”, paving the way for interceptions and the turning back of boats, in a defiant statement of intent.
The move came after many more migrants, including one in a wheelchair, were brought ashore from a Border Force boat on another busy day for Channel crossings.
More than 130 migrants succeeded in making the perilous journey to the UK on Friday – part of a surge Ms Patel condemned as “appalling”.
More than 4,000 migrants have now reached the UK this year by crossing the English Channel in small boats, more than 650 so far this month alone.
Jack Straw, the former Labour home secretary who wrestled with the same problem 20 years ago, led criticism of turning to the navy in a bid to “push these people back”.
“It will only take one of these dinghies to capsize and everybody to drown – which is perfectly feasible – for there to be a hullabaloo, including in the Conservative Party, and for the policy to have to be reversed,” he warned.
Bella Sankey, director of the Detention Action human rights group, condemned the “unhinged proposal”, warning the home secretary it would be met with legal challenges.
“It’s unlawful, it’s really dangerous and could seriously risk human life,” she said. “From any way you look at it it’s a terrible idea, and I don’t think it would actually get off the ground.”
In increasingly hardline language, the immigration minister Chris Philp called for the migrants to be fingerprinted and face “real consequences”, before a meeting next week with his French counterparts.
But Ms Patel was also warned that the completion of a hard Brexit would make her attempts to persuade France to take back more of those making the crossing even more difficult.
Colin Yeo, an immigration barrister, pointed out that the current arrangement – the Dublin regulation – is not being renegotiated. “It will become harder to return migrants to France in 2021, not easier,” he said.
The new UK tactic, modelled on Australia’s approach against migrants from Indonesia, would see both the navy and Border Force intercepting vessels after they leave French waters.
But Tony Smith, a former Border Force director general, said only an agreement with the French would stop dangerous crossings, because “the law of the sea kicks in”.
Migrants have threatened to jump overboard if they are turned back, which would require UK ships to withdraw because preserving life is paramount under the law of the sea.
Nevertheless, Ms Patel announced the appointment of ex-marine Dan O’Mahoney with responsibility for “making the Channel route unviable for small boat crossings”.
“We are working to make this route unviable and arresting the criminals facilitating these crossings and making sure they are brought to justice,” she vowed.