By Andrew Bomford
BBC Radio 4 PM reporter
Customs officers are seriously concerned about security at regional ports and airports because many ports are left unmanned most of the time.
Drug seizures have dropped, according to customs sources
A BBC investigation has learnt that mobile teams of customs officers are being diverted to duties at ports like Dover, depleting staff elsewhere.
Miles of coastline in the southwest and Wales are left vulnerable as a result.
The Revenue and Customs said it was focussing on improving intelligence, even while leaving ports uncovered.
Sources in the service told the BBC that seizures by customs officers of hard drugs and firearms have fallen dramatically.
"You may get the odd seizure, " said Michael, (not his real name) a recently retired customs officer from the Plymouth area.
"But it's just the tip of an iceberg. The vast majority of importations go unchallenged, simply because there's no staff here."
In 2003 customs officers moved to a system of mobile team working.
Permanent staff at ports like Plymouth were transferred onto mobile teams, with the idea that they could move quickly to wherever intelligence suggested the threat is.
But in practice, according to customs officers who have spoken to the BBC, and their union, the Public and Commercial services Union (PCS), mobile teams spend most of their time working at the busy ports and airports in the South East of England, like Dover, Heathrow or Gatwick.
"At a guesstimate, you'll be lucky to get a team of customs officers here in Plymouth one day a week," said Michael.
"The criminals all know this," he added.
"They just put the word out - the customs officers are down here, so go somewhere else. You'd have to be pretty thick not to work it out, and these are intelligent people."
Mobile units tend to spend the most time at South East of England ports
Another Customs officer who worked on the mobile teams told the BBC: "The bulk of the time is spent in the south-east. They might do the odd Friday in Plymouth to cover the Spanish ferry - but the Spanish ferry comes in twice a week."
Asked whether there was ever specific intelligence about a smuggling operation on a particular ferry but there were no customs officers to respond, he said it happened quite often, and that there are regular examples of hard drugs being found around the coastline of the south-west by children.
In September 2005 a notorious Essex drugs dealer was spotted sailing into the picturesque Salcombe Harbour on the south Devon coast by yacht.
The police had been alerted by a local resident who spotted a suspicious car, but the drugs dealer dived overboard and swam across the harbour to escape.
Police found £100,000 worth of amphetamines.
In another incident in June 2005 £1 million worth of cocaine was found in a remote cove not far from Salcombe.
Chris Stidston-Nott, who runs a security company patrolling Salcombe Harbour, said it was impossible to check every boat coming into the port because of the sheer numbers of yachts and pleasure craft.
"It's not easy," he said.
"During the main season we get a lot of boats coming in and out, and leaving in the early hours.
"So if we are in one part of the harbour and something happens somewhere else, we are going to miss it."
According to the PCS union, which represents 84,000 staff working for HMRC - Her Majesty's Revenue & Customs - there have been no customs seizures of hard drugs or firearms in Devon and Cornwall since the start of mobile team working in 2003.
Previously, Customs officers based at the port of Falmouth had seized the largest numbers of firearms and ammunition of any port in the country.
Martin Peach, head of detection at HMRC, told the BBC that Customs did not want to leave any port or airport uncovered, but the emphasis now was on improving intelligence.
"I am happy that we have as good an intelligence picture as we have ever had of the risk in the south-west and elsewhere."
"A thin blue line strung along the coast is no longer the answer," he said.
"Intelligence is the answer."
Lord Carlile, the UK's Independent Reviewer of Terrorism Legislation, told the BBC he was concerned that security at regional UK ports and airports was being compromised.
He described it as "ludicrous" that mobile customs officers are working out of their home areas.
"If you compare the structure of the Customs and the way they deal with possible terrorism, and the structure of the police, you do actually see that the regionalisation of the police has been very effective," said Lord Carlile.
"Now of course Customs are involved in this activity, but they just aren't there when they may be needed and that's very unsatisfactory."
"In my view it would merit the attention of a Select Committee of the House of Commons in perhaps roughing the Customs up a bit," he added.
Mr Peach said he wanted to talk to Lord Carlile about his comments.
"I don't believe security is being compromised," he insisted.
The PCS union is also increasing pressure on Customs, by preparing a dossier to go to all MPs, detailing various allegations about a lack of customs cover at regional ports and airports across the UK.