Rail staff failed to spot a "fake bomb" planted on a train by undercover Department for Transport (DfT) investigators.

A suspicious package was placed on a train to test if Govia Thameslink Railway (GTR) staff would spot it. They did not. Govia Thameslink is the parent company of Southern – the rail network beleaguered by strikes that have prompted calls to renationalise the operator.

In a memo leaked to The Times, Tony Holland, the crime and security manager of GTR, disclosed that staff failed the anti-terrorism test. Unions have sought to capitalise on the incident and argue that it highlights the risks inherent in proposals to remove conductors from Southern trains. But Southern hit back accusing the unions of grossly misrepresenting the rail company's plans to modernise the services.

Holland's memo read: "Last week transport security inspectors from the Department for Transport carried out a covert test on one of our trains.

"The test involved leaving an unattended bag on a train, which contained a suspicious item. Disappointingly, the bag was not found despite it being positioned in a public area of the train, and railway personnel seen to walk past the location.

"This incident further highlights the need for us to be alert and identify any unattended items that may be left on trains and stations. Please be reminded that we all have a part to play in keeping our railway safe, regardless of our roles. Please remain vigilant at all times when on or about our trains, stations and depots."

A spokesman for Aslef, the train drivers' union, told The Times: "It is quite extraordinary that Southern Railway has failed a DfT security test while getting rid of the guards on the trains. How does this help reassure passengers?"

Aslef and GTR have been at war over plans to run driver only operated (DOO) trains without a conductor. GTR argues that, while DOO trains will require drivers to operate carriage doors, there will in fact be more personnel on board with passengers during journeys.

A spokesman for GTR said: "Our modernisation of Southern's train service means there will be more people working on our trains, not fewer, and they'll be able to give better customer service as well as looking for suspicious bags.

"It's only while the unions carry on with this entirely unwarranted industrial action that we have been forced, on occasion, to run a train with only a driver which is better than cancelling the service as we had to do before we changed to driver-controlled operation.

"All railway staff are acutely aware of the increased security threat and we pass the overwhelming majority of DfT's routine tests and investigate any that we fail."