Soldiers and sailors covering for striking Border Force staff at passport control do not have the power to detain people they suspect of criminal activity, leaked documents show. Emails reveal that people suspected of crimes such as carrying a false passport, drug smuggling, people trafficking and victims of modern slavery cannot be stopped by members of the armed forces if they hold valid travel documents. Instead, a separate intervention has to be sought for suspected serious criminals or their victims from a fully trained Border Force officer, most of whom are currently on strike. One Royal Navy officer warned in the emails that the inability to detain might “impact op [operational] capability during industrial action”. The disclosure goes some way to explain why there has been little disruption at UK ports and airport since the PCS strike for improved pay and conditions began on Friday. One email, sent on 22 December, shows that a Royal Navy sub-lieutenant wrote to Border Force and Home Office civil servants asking whether Royal Navy staff working at Manchester had the power to detain suspects. He wrote: “RN personnel currently deployed at Manchester airport are currently prevented from completion and issue of IS81 and must not be involved in detention activity, referring to a BF officer. This may impact op [operational] capability during industrial action.” An IS81 form gives immigration officers the authority to detain people while they undertake further inquiries, according to Home Office documents. It has to be issued even if someone is detained for just two minutes for a minor check. A senior Border Force officer replied, saying that armed personnel had not been given enough training to detain suspects. “Received email below following my discussion with C2 Navy lead at Manchester. They have said that they cannot issue an IS81 and this has to be done by a Border Force officer as there was only training up to IS81. Can this be clarified as a matter of urgency,” he wrote. Another senior Home Office official confirmed in an email: “This is correct. I have communicated this previously. At the point at which any military personnel need [sic] are not able to land a passenger a permanent member of staff will need to issue an IS81 on their behalf.” About 600 armed forces personnel and 200 civil servants from the Home Office have been brought in to cover for Border Force officers during the strike, which is due to continue every day of December except for 27 December. They were given five days of training and have been brought in to take people’s passports and check them against the “warnings index [WI]” – a Home Office watchlist database with information such as previous immigration history and matters of national security. Border Force guards are usually given three weeks of training as a minimum before they interact with the public. After the three weeks, they are given a mentor to work alongside for up to a month to ensure they can work solo on a passport desk. While in training, they learn how to process and interview passengers, identify victims of modern slavery, spot forged documents, identify suspected trafficked children, and learn how to question passengers. The PCS general secretary, Mark Serwotka, said: “The government has boasted there are no queues at passport control, but of course there are no queues if no one’s being stopped. “And no one’s being stopped because the government has adopted a sticking plaster approach to this problem. “We warned before the strikes started that military personnel with five days’ training wouldn’t be able to do the jobs of experienced, professional Border Force officers. “We’ve made it abundantly clear the government can stop this dispute tomorrow by putting some money on the table.” There had been “very few” fully trained Border Force guards on hand at Manchester to help oversee dozens of army officers checking passports, sources said. Sources from Heathrow airport have informed the Guardian that members of the army covering at Heathrow have been told that they cannot detain people. A Heathrow staff member said: “The covering soldiers have been told not to stop anyone unless evidence of criminality is flagged up [through the WI]. In terms of immigration issues, they are being told to land everybody.” A Home Office source said Border Force officers who were not on strike were continuing to work alongside members of the armed services and other contingency staff. “We want to minimise disruption throughout this dispute,” the source said. A Home Office spokesperson said: “Border Force has developed robust contingency plans which prioritise keeping our citizens safe and our borders secure. “Non-striking Border Force staff, with the full range of appropriate powers, are continuing to complete their vital roles and we have spread our resources flexibly to ensure there is sufficient cover to fulfil our key priorities. “Military personnel, civil servants and volunteers are supporting a range of services and all deployed contingency staff are sufficiently trained for the activities they are required to undertake.”