The development has concerned permanent staff in Dover who believe the unprecedented move could compromise security.
It comes as the government said it will recruit up to 1,000 Border Force officers to deal with expected delays if Boris Johnson fails to strike a deal with EU leaders by 31 October.
Border Force staff in Dover have in recent weeks held a series of meetings with managers and Home Office officials. They have been told that dozens of agency workers will be used to help in the “facilitation of the movement of goods” away from port terminals if the UK leaves without a deal.
Border Force guards are expected to oversee the movement of goods in freight lorries away from ports as well as checking immigration status, searching cargo and gathering intelligence.
A senior staff member said: “The introduction of agency staff hasn’t happened before in this way and there is a risk. We are expected to keep an eye on vehicles in transit in case they are transporting illicit goods and illegal immigrants. Placing people with little loyalty to the job leaves obvious possibilities for exploitation.”
Diane Abbott, the shadow home secretary, said the disclosure was “deeply troubling”.
“Proper protection of our borders from all types of smuggling, people and drug-trafficking requires training, experience and co-ordination. The government is providing none of this.
“Boris Johnson promised more secure borders. But with his reckless aim of crashing out the EU with no deal, his government is actually creating the opposite,” she said.
A total of £344m will be spent on new border and customs operations, including doubling the support for customs agents.
Over 2018-19, the average number of Border Force guards was 8,197. Although the government is promising another 1,000 staff, this is still 1,000 short of the number needed to handle a no-deal Brexit according to the National Audit Office last year.
Union officials have expressed concern about whether the training received by new recruits will be thorough if they are being rushed through before the end of October.
Staff in Dover have been told that immigration enforcement officers, who usually work away from the UK’s borders, and HM Revenue and Customs officials could also be brought to Dover to work alongside Border Force staff in the event of a no deal.
The Home Office has also said there will be funding to improve transport infrastructure around ports and for Operation Brock, the traffic management plan in Kent which involves motorway closures and the use of Manston airport as a giant lorry park.
As Europe’s busiest ferry port, Dover is used by 120 ferries and 90,000 passengers each day. It also handles more lorries than all other UK ports combined.
The Border Force is recruiting extra officers at more than 20 ports and airports as part of preparations including Dover, Plymouth, Grimsby, Poole, South Shields and Pembroke.
A statement on the Home Office website said: “Security will always be our top priority and, with both data and intelligence, we have a clear picture of those wanting to enter the country.
“It’s about blending this knowledge with the skill and intuition of our officers to make informed, swift decisions.”
The Home Office has been contacted for a response.