Migrants are believed to have unlocked a gate into the Eurotunnel complex after guessing the security code, it has been reported.
It comes after David Cameron said a lot more needs to be done to bolster security to protect Britain's border amid the continuing migrant crisis in Calais.
The Daily Telegraph said it has photographs revealing how a group of 30 migrants breached security and walked inside a secure zone.
The men walked up to a gate about a mile from the Coquelles Channel Tunnel terminal near Calais and were inside within seconds, the newspaper reported.
It is thought they were able to guess the security code by examining which numbers were dirtiest or most worn on the keypad.
The newspaper said a close examination showed the numbers two, four and zero were clearly more worn than the other numbers.
The Home Office said it does not comment on specific incidents, but a spokeswoman said: "We continue to work closely with the French government and Eurotunnel to tackle the immediate pressures and longer term issues involved in the situation in northern France.
"The UK Government has recently invested £7 million for fencing at Coquelles to secure the Eurotunnel platforms, as well as further perimeter fencing.
"The first phase of fencing at Coquelles was completed today. Extra Border Force search and dog teams have also been drafted in to further protect the Eurotunnel site.
"Since last year, the UK Government has provided funding to bolster the security and infrastructure of ports in Northern France — including a £2 million upgrade of detection technology, a £1 million extension of dog searching capacity and an additional £12 million to reinforce security at the juxtaposed border."
Earlier, footage and pictures, shot by the Home Office, showing new fencing installed in Calais and Coquelles was released.
It is hoped the fencing will secure one mile of Eurotunnel's shuttle platform area where migrants attempt to board waiting and slow-moving trains destined for the UK.
Meanwhile, although more fencing and police officers had helped, further improvements to halt migrants crossing illegally into Britain were taking place "in the coming weeks and days", the Prime Minister said earlier.
Mr Cameron said: "We have done a lot in recent days to improve the situation but there's a lot more to do.
"So we have got more fencing, we've got more police officers, more sniffer dogs, more guards, better security, and we are making progress.
"But there's a lot more to do, including better security in the tunnel itself and myself and the team of ministers I have put in place, we will oversee these improvements and they will take place in the coming weeks and days."
His comments came as it was revealed that ministers had discussed plans to close the 31-mile Channel Tunnel if the crisis in Calais worsens.
The proposal has been discussed at the Government's emergency Cobra meetings in what has been described as the "nuclear option", the Daily Telegraph reported.
Sources said the option to close the Channel Tunnel - operated by Eurotunnel - exists but there were no immediate plans to do so.
A Government spokesman said: "We have and continue to consider all potential courses of action to improve security at the Channel Tunnel in Coquelles and to prevent any loss of life.
"A number of new security measures have been introduced and ministers both here and in France keep the situation under constant review."
The disclosure came as it emerged a suspected illegal immigrant from Sudan walked nearly the entire length of the Chunnel from Calais before being held by British officers.
He dodged hundreds of security cameras and officers at the Calais entrance before reportedly being spotted by British security guards 11 hours later near the Folkestone exit on Tuesday evening.
The intrusion has been described by Eurotunnel as "extremely rare" and "extremely dangerous", with trains travelling in the tunnels at up to 100mph.
Kent Police said Abdul Rahman Haroun, 40, has been charged with causing an obstruction to an engine or carriage using the railway under the Malicious Damage Act 1861.