Tighter security in and around Eurotunnel's French terminal over the past few weeks has prevented a repeat of the mass incursions seen earlier this summer but migrants are ready to exploit weak spots as sporadic disruption to truck shuttle services this week testifies.

Overnight between Wednesday and Thursday, migrants intruded on to the site from an SNCF marshalling yard at Frethun where long-distance international freight trains stop before heading to the UK.

"The Frethun marshalling yard's tracks eventually join those of Eurotunnel and migrants have been quick to exploit this weak spot in security. Earlier this week, we had a series of intrusions via the yard, in a single night and had to suspend truck shuttle services for brief periods. Until this issue is addressed we can't rule out further incidents of this kind," a spokesman for Eurotunnel told Lloyd's Loading List.com.

"We have been on to SNCF for some time to tighten up security at the yard where trains change locomotives before going through the Tunnel. This gives migrants the opportunity to stowaway on freight trains before departure or jump on to them as they leave the yard. When they don't succeed - the trains are subject to Customs checks and also security inspections just before departure - they move on up to the Eurotunnel terminal, ? the spokesman explained.

An agreement has been reached with SNCF for Eurotunnel to erect two kilometres of high-security fencing around the yard's perimeter as soon as possible, he added.

"That the marshalling yard has become a popular route for migrants is a 'change of tactics' on their part, indicating that the measures to heighten security elsewhere at the Eurotunnel site this summer - in particular, a significant extension of high-security fencing but also more sniffer dogs and a greater police presence - are having the desired effect."