A key government adviser on terrorism has warned there are not enough customs and immigration officers at many sea and airports.
Lord Carlile is reviewing the operation of the Terrorism Act
Lord Carlile told BBC Radio 4's File on 4 officers were "spread a little thinly and that's not entirely satisfactory".
He said nobody could feel totally confident all terrorists were likely to be stopped from entering Britain.
The Home Office says the immigration service ensure all arriving flights are assessed and appropriately handled.
Customs also say they play their full role in countering terrorism.
Both the immigration service and customs follow the policy that officers are deployed where risk is thought to be greatest. This means that for parts of the day, small and medium-sized ports of entry may have no officers at all on duty.
Lord Carlile, whose role includes reviewing the operation of the Terrorism Act, criticised the level of cover of both services, and told File on 4 that at some ports, a customs officer was a rarity, and immigration officials might also be absent.
He questioned whether there were enough officers to protect the country fully against terrorism.
Lord Carlile told File on 4 he believed most terrorists were likely to be caught.
However, he added: "Nobody could put their hand on their heart at the moment and say they feel totally confident that all terrorists are likely to be caught coming in from abroad."
On the programme, the Liberal Democrat MP for Southport, John Pugh, challenged the level of immigration service cover at Blackpool Airport. He said he was told by a security source it was "wide open".
Mr Pugh also told BBC Radio Five Live tha he was concerned about security at regional airports.
"I'm not suggesting that regional airports are haemorrhaging people right into the country in vast numbers," he said.
"But I am suggesting there are a significant number of people creeping in whose identity we do not know, who probably in most cases will work in the black economy, but who may have more dangerous motives in coming here."
John Tincey, Vice Chairman of the Immigration Service Union, told File on 4 there were gaps in Britain's national defences.
"It's possible that there are, in a number of the very small airports around the country, flights arriving when immigration staff are not available, but that is because they are deployed at other locations meeting other aircraft where the risk is considered to be greater," he said.
"It's not a situation I'm happy with."
File on 4 is on BBC Radio 4 at 2000 GMT on Tuesday 14 March