The body of a teenager was discovered on a Eurotunnel train at the Channel Tunnel terminal in Folkestone.
Kent Police said the male body was discovered on the loading bay of the vehicle shuttle service in the early hours of yesterday morning.
It is thought to have been a migrant trying to reach the UK from Calais.
Eurotunnel had blamed delays to journeys through the tunnel on “intensive migrant activity” and “very high levels of traffic” leaving the UK.
It reported delays of up to two and a half hours to its shuttle passenger service from the UK yesterday.
Eurotunnel said this morning: “Our service is operating with some delay due to overnight migrant activity. Please be assured we are working hard to resolve this.”
Operation Stack, in which lorries queue on the M20, has been extended to junctions eight to 11 coast-bound.
Kent Police said it was likely to be in place until at least Sunday, with a busy weekend expected for Channel ports with people leaving on holiday and strong winds and heavy rain forecast for this afternoon and Saturday.
The Port of Dover advised travellers to check with ferry companies for updates on crossings to France.
P&O Ferries warned of high volumes of traffic this weekend and advised passengers to allow extra time for their journeys.
Clia UK & Ireland is holding a luxury cruise expo in the port this weekend with Crystal Cruises and Windstar.
The French government has unveiled proposals to save ferry jobs in Calais and end a stand-off in the port in which 500 ferry workers have blocked services.
The workers are protesting after MyFerryLink, previously owned by Eurotunnel, was sold to DFDS Seaways.