The RMT general secretary has defended rail strikes across the UK over Christmas and said unions have a “duty to coordinate what they do”, saying the government is sending a message that pay rises will only come with worse terms for workers. Mick Lynch said the government was “coordinating … an attack on working people” by changing working conditions and offering below-inflation pay rises. “It would be foolish of unions not to coordinate themselves in response to those attacks,” he told BBC Radio 4’s Today programme on Tuesday. Lynch said the strikes over Christmas would “not largely affect passenger services” but would have an impact on engineering works between 24 and 27 December. On Monday night, the union announced the additional strike dates and rebuffed a pay offer from Network Rail shortly before the industry’s deadline. The RMT said it would put the offer to its members in an electronic referendum this week but recommend that they reject it. It affirmed that two 48-hour strikes that would stop much of the railway next week would go ahead in any case. Lynch said all passenger trains on Christmas Eve would run until the evening, but strikes would then begin to affect services. “It is a change of plan because the companies have changed their plans,” he said. He said the timing of the strikes was “unfortunate, but we have to respond to what the companies are doing, and they’re doing that very deliberately. They’re seeking to ratchet up the dispute.” He said train companies had threatened to impose changes to operations, including potential job losses in the future. “If we do not respond, then those changes will go through without a response from us and our members will have to suffer the consequences including job losses and changes to their working lives that are unacceptable to them. One of those things are more unsocial hours or more weekend working.” The schools minister, Nick Gibb, said the strikes would “hold the country to ransom” over the festive period. He told GB News: “It’s a very disappointing decision by the RMT. They were offered a very good pay deal by the employers, 8% over two years, which is in line with the kind of pay deals that are taking place outside the public sector. “We would urge the unions to talk to employers, to keep negotiating and not to hold the country to ransom, particularly in December as we get nearer to Christmas.” Labour’s deputy leader, Angela Rayner, told BBC Breakfast that rail workers would not have taken the decision to strike lightly. “These people who are going on strike are going to lose pay, they will lose their pay at a time when they will need it most. They are not doing it at a drop of a hat,” she said. “This is a militant government that is not dealing with the issues and not resolving this strike action, and it’s frustrating. The system is absolutely crumbling without the strikes. Anyone who gets on a train now in the north knows that you’re praying if you’re going to get to where you need to get to.” The row over the rail strikes came as the UK Statistics Authority, which provides independent regulation of all official statistics produced in the UK, said it was investigating how train cancellation statistics were reported, following a series of Guardian stories. Some train companies – with TransPennine Express (TPE) the worst offender – are taking advantage of a loophole that means cancellations do not “count” in official statistics if they are announced by 10pm the night before. They use something called a p-code, originally designed to allow companies to cancel trains for reasons beyond the operators’ control, for example after a landslide. But the P-code is open for operators to interpret, meaning some only use it for circumstances that are not their fault, while others, such as TPE, use it when they simply do not have enough staff. The result is misleading data that only includes on-the-day cancellations. TPE cancelled about 20% of services last month but will only have to report on-the-day cancellations of about 6%. The latest strikes will take place from 6pm on Christmas Eve until 7am on 27 December, curtailing some of the last passenger trains before Christmas and potentially disrupting a wide programme of engineering works on the railway. Most trains do not run on 25 or 26 December. Apart from the Christmas bank holiday weekend, strike days were already planned for 13-14 and 16-17 December, then on 3-4 and 6-7 January, with widespread disruption expected on the days after the strikes. However, the RMT has called off industrial action short of a strike at Network Rail, including an overtime ban that insiders said could have caused even more widespread disruption. Some had feared that parts of the rail network would have been left almost as stretched during its planned work-to-rule period as on the eight days its members are scheduled to strike. The RMT’s national executive committee met on Monday afternoon to consider an improved offer from Network Rail that raised pay for 2022 – backdated to January – by 5%, with a further 4% at the start of 2023. The smaller TSSA union said it was calling off strike action in December while it balloted its members at Network Rail on the improved offer. This article was amended on 6 December 2022 to make clear that the RMT halted an overtime ban only for Network Rail members.