Britain's coastline is facing one of its biggest ever breaches of borders as an unprecedented number of small boats and dinghies smuggle in migrants, a top union official has warned.

It comes after 18 Albanians had to be rescued when their inflatable boat sank off the coast of Kent on Saturday night.

Lucy Moreton, general secretary of the Immigration Services Union, said large stretches of Britain's coastline is being left unpoliced and officials simply do not know how many people have sneaked into the country undetected.

But she told the Press Association her "gut feeling" and anecdotal evidence suggest Britain's coasts are facing the biggest ever onslaught of people smugglers.

She said: "The easy answer is this is the worst we have ever seen it, and it is insofar as this is the worst that has ever been recorded. But we have no way of knowing because we didn't look for this until recently.

"My gut feeling is yes - this was an inevitable progression, once you saw the floods of people coming up from north Africa through Turkey into southern Europe and making their way up through Europe.

"That was always going to mean that whatever irregular route they used, whether it was climbing into containers, hanging on to the bottom of cars, clinging on to trains, walking through tunnels or getting into a small vessel - all of that was going to increase.

"And our in-country asylum applications numbers are rising, so the statistics suggest that this is a massive increase."

Britain has three Border Force vessels to patrol more than 7,000 miles of coastline.

Ms Moreton said resources have been focused on beefing up security at Britain's airports and on road and rail trafficking routes out of Dunkirk and Calais.

Meanwhile, "we don't routinely police coastlines at all", she said.

Faced with a crackdown on roads and railway tracks, an increasing number of migrants are resorting to making the perilous journey across the Channel by inflatables instead.

Ms Moreton said: "Wherever we tighten security in one place there will be another vulnerability. We tightened the security massively to stop people climbing into lorries and under cars, all those resources were thrown into the Channel ports, into Calais, into Dunkirk.

"That made those crossings become more difficult and it pushed that traffic elsewhere. They will go to smaller French ports and make container type crossings, or they will go and buy inflatables and try to cross the Channel.

"The problem with trying to cross the Channel in a small inflatable is this is one of the busiest shipping lanes in the world and these small vessels cannot be seen by a big vessel before it runs over them.

"Crossing the Mediterranean is dangerous, crossing the Channel isn't that much better."

A spokesman for the Public and Commercial Services Union, which also represents border staff, said: "We've said for years that there aren't enough staff at the borders and we've been ignored and vilified by Government ministers and senior officials.

"Instead of pretending everything's fine, moving staff to cover passport gates and leaving gaps elsewhere, the border agency needs to get round the table with us to discuss rationally what resources they need and where."

It follows similar warnings by former senior Royal Navy and immigration figures who said Britain is at risk from terrorists and traffickers because of the lack of boats that patrol UK waters.

Former head of the Navy Lord West told the Daily Mail: "It is a complete mess. We are taking a calculated risk with our own territorial waters.

"Already we have seen these illegal immigrants and I don't believe there aren't clever traffickers using the smaller ports to send them, and I'm sure terrorists are aware of the route too.

"We need to get a grip on this. We are taking a gamble that nothing will ever happen in our seas and that is a risky view to take given the dangerous world we are in."

The National Crime Agency, Britain's top crime-fighting force, last month warned that criminal gangs have begun targeting quieter ports.

Tom Dowdall, deputy director of the NCA's border policing command, said smaller ports on the south and east coasts have been targeted in addition to the key hotspot of Kent.

He said: "We've seen on the east coast evidence from Tilbury and Purfleet, up as far as Hull and Immingham, and on the south coast from Newhaven to Portsmouth."

New maritime enforcement powers will come into force on Tuesday which will allow Border Force officers to stop, board, divert and detain vessels, and arrest anyone they suspect has broken immigration laws.

The stronger maritime powers, announced by the Home Office last week, are intended to crack down on illegal immigration and those trying to sneak asylum seekers or those in breach of a deportation order into the country.

Three maritime co-ordination hubs in Cornwall, the Thames Estuary and the Humber over summer are also being set up.

Retired naval officer Rear Admiral Chris Parry said the Government should deploy a warship to the Channel rather than lending it to the Mediterranean operations.

"The answer is to put more resources into a comprehensive surveillance and intercept system that will enable us to pick up every vessel that is either transiting or coming across from the continent," he told BBC Radio 4's PM.

"We also need co-operation with continental partners to make sure that the sending countries are able to provide that level of security while we look out for our maritime borders.

"The Royal Navy is extremely stretched around the world. The Prime Minister at the G7 conference very generously gave a warship to help the effort in the Mediterranean.

"Maybe we should be putting that warship in the English Channel to help coordinate the surveillance and intercept capability.

"What we have to do is encourage our European partners to pull their weight.

"If we have a compelling need in the Channel to defend our maritime frontier then the resources have to be put there whilst doing our best to contribute to what is going on in the Mediterranean."

He said: "Unless we take action we are not going to stop people coming across the Channel in increasing numbers as the summer progresses.

"Most people who know how to use a boat could get across the Channel undetected and our border with France along the maritime coast there is extremely porous.