A Reform UK proposal to use the military to stop small boats bringing migrants to the UK from France lacks substance, a former Royal Navy officer has said, suggesting the UK’s warships would be better used countering actual military threats.

Reform’s political opponents have claimed its plans have more to do with distracting from the financial scandals surrounding its leader, Nigel Farage, than about solving the small boats issue, after the party proposed having the navy pick up people trying to make irregular Channel crossings and take them to France.

“A billion-pound warship that’s configured for anti-air warfare or anti-submarine warfare, which are both threats to the UK, is not best used ferrying people around. You’re better off using an actual ferry,” the retired Royal Naval Cmdr Tom Sharpe told BBC Radio 4’s Today programme on Monday.

Sharpe said Reform’s proposal “lacks quite a lot of substance” and that the navy did not have suitable ships to spare, given the military operations it was already carrying out.

He added that the proposal seemed very similar to a plan put forward by the Conservatives four years ago, while that party was in government. During that time, crossings almost doubled, the navy was said to have been deployed as a “taxi service” and the government handed millions of pounds per year to private firms to pick people up in the Channel.

Labour said Reform had “reheated an old announcement in their latest desperate bid to distract from their growing sleaze scandals”. A spokesperson said: “Anger and easy answers won’t bring crossings down. The serious action we’ve taken have started to have a real impact and has led to 46,000 attempted crossings stopped, and 70,000 people with no right to be here removed.

“We know there’s more to do on this complex issue. What is far less complicated is Nigel Farage’s brazen attempts to dodge scrutiny over his secret £5m gift from a crypto billionaire. It’s time he stopped the distraction tactics and came clean with the public.”

Sharpe said one of the biggest hurdles was the likelihood of the French authorities agreeing to allow the navy to dump people on their shores. Speaking to Today on Monday, Reform’s spokesperson for home affairs, Zia Yusuf, said: “If the French disagree, what will end up happening is we’ll end up having something of an argument with the French.”

He claimed that French authorities refusing to allow people to disembark on its shores would be a “crime against humanity”. Yusuf added: “This is a boat full of innocent civilians who are being rescued as part of a humanitarian mission.” He did not explain how his party’s policy of refusing to allow any to make it safely to the UK’s shores would not constitute the same.

Yusuf claimed Reform had sought legal opinion that its plan would be lawful, saying it would define people making the Channel crossing by irregular means as being in distress at sea – obliging the navy to rescue them. He said his party would also tell French authorities they must abide by obligations under international law to process those people’s asylum claims “because it is a safe country”. He did not explain what Reform’s stance would be if France simply said the same to the UK.