A work-to-rule strike by French customs officers warning they are not ready for Brexit is likely to stretch into next week and cause further chaos on Eurostar trains and channel ports.

At Gare du Nord in Paris on Wednesday, hundreds of passengers queued in lines stretching the full length of the station, with an estimated six-hour wait for trains to the UK. Eurostar has encouraged people not to travel at all between Paris and London this week if they could avoid it.

French customs officers are into their third week of industrial action over long-running anger at working conditions, saying a lack of preparation for a no-deal Brexit was the final straw.

Instead of walking off the job, customs officers are diligently carrying out longer and more rigorous checks than usual, to demonstrate what might happen if full border controls are put in place after Brexit, particularly in the case of no deal.

Union leaders said customs officers at the Eurostar terminal were simply asking two or three extra questions of passengers – which was causing acute delays and train cancellations.

When the action began at Calais and Dunkirk ports on 4 March, customs officers simply carried out vehicle checks and requested registration documents, resulting in two weeks of lengthy tailbacks for trucks waiting to cross to Britain. Unions warned the disturbance caused by rigorous checks was a taste of post-Brexit reality.

Vincent Thomazo, secretary general of the customs branch of the Unsa union, told the Guardian: “This strike is over long-running concerns over our general working conditions, but Brexit was the final straw. We believe that contingency plans here for Brexit are low-cost, with not enough extra customs officers. Brexit has highlighted a sense of angst that had been there for years.”

Of a no-deal Brexit, Thomazo said: “We are absolutely not ready – no one is. Everyone is in denial.”

The French government insists it is ready for a no-deal Brexit, with the recruitment of an extra 700 customs officials. Unions say more are needed.

Last week French unions representing around 17,000 customs workers rejected a government offer of a €14m payroll boost, saying it was insufficient. No new talks are scheduled, and the joint industrial action by all unions is open-ended.

In the Eurostar queue at Gare du Nord, Ashley Barnes and Zach Mance from Portland, Oregon, were bracing for a six-hour wait. The US couple had deliberately booked their honeymoon trip between Amsterdam, Paris and London before the Brexit exit date of 29 March to avoid travel chaos.

“We thought since we’d be there before Brexit, it would be fine,” said Barnes. Mance, who works for a small winery in the US, had – like many in the queue – brought a bottle of wine to share for the wait.

Keith Howell from Essex, who used to work in banking, and Helen Aslett, who works for a small firm in Braintree, had worried more about the gilets jaunes (yellow vests) protest in Paris when booking their trip. They had rushed to visit Montmartre and the Moulin Rouge in the morning before dedicating almost a full day to queuing. They had voted to remain in the Brexit referendum. “I didn’t think we had enough information or knowledge to vote leave,” Howell said. “But we had been told an exit would be orderly, instead there is complete uncertainty.”

“Even more reason to get away from France!” said Nick, 64, a retired healthworker from Essex. He had voted to leave the EU but feared if Brexit was postponed it might never happen. His partner, celebrating her birthday with a three-day trip to Paris, had voted remain, “because I feared that leaving would be a mess”.