Transport secretary Grant Shapps says he is considering a change in the law to allow agency staff to take the place of striking railway workers, but the prospect has hardened the unions’ stance.

Any change could not be achieved in time to affect the various strikes planned for later this month, including three 24-hour walkouts on National Rail in England on 21, 23 and 25 June, but could make further industrial action less effective.

Mr Shapps told the Sunday Telegraph that if walkouts continued, ‘further measures certainly would come in during this particular dispute, if it can't be resolved’, by using ‘secondary legislation’ which could be brought in quickly. He continued: ‘We will be looking at the full suite of modernisation that's required. The country must not continue to be held to ransom.’

RMT general secretary Mick Lynch responded: ‘Grant Shapps needs to stop smearing the RMT and unshackle the rail operating companies so they can come to a negotiated settlement that can end this dispute. Railway workers voted overwhelmingly for strike action in defence of their jobs and for a pay rise that deals with the rising cost of living. It is insulting to them to suggest they do not understand the issues that affect their daily lives or cannot make a democratic decision by themselves.

‘We already have the most restrictive anti-democratic trade union laws in Western Europe and if the government attempts to reduce our rights further, the RMT along with the rest of the trade union movement will mount the fiercest resistance possible.’

ASLEF general secretary Mick Whelan said: ‘You can't just ship in a load of signallers and train drivers: If they are not working for the railway now they are either not medically fit, have been dismissed or don't want to do it any more.’

Unison’s general secretary Christina McAnea added that her union would be ‘campaigning – doing more than campaigning – to make sure that it doesn't happen.’

Apart from the walkouts on Network Rail throughout Britain and at 13 English train operating companies, separate action is planned by ASLEF, which has announced strikes of its driver members at Greater Anglia on 23 June, which will coincide with an RMT national strike. White collar union TSSA is balloting its members at Avanti West Coast, and a walkout there could affect operational managers as well, who will not then be available to stand in for striking staff.

London tram drivers are staging two 48-hour strikes on 28-29 June and 13-14 July, while open access operator Hull Trains faces disruption on 26 June, when its drivers will walk out for the day. London Underground is also set to be disrupted by another strike on 21 June.

The prospect of still more disruption is also growing, because ASLEF is balloting its driver members at nine other operators.