Train operators are making plans to restore around 80% of services from next month, should the government decide to partially lift lockdown measures in May.

Rail unions have said they will tell members to stop work unless the contingency plans address safety concerns.

Passenger numbers on trains are about 5% of normal levels, with only key workers supposed to travel. Most parts of the network are operating Sunday levels of service throughout the week.

However, with the rail industry requiring four weeks’ notice to amend schedules, preparations are starting for a potential Saturday-style service, roughly 80% of weekday timetables.

The government and operators are understood to be aiming to be ready to add capacity as early as 18 May. The summer timetable was due to come into effect on this date, requiring changed rosters and route training, regardless of any lifting of coronavirus restrictions.

Some operators are uneasy about a swift restoration of services. One source said trains could be no more than 20% full with social distancing rules. “The industry will take a hit if this is not well managed and too many passengers are onboard. Who is allowed to travel, should they be distancing and wearing masks, and who enforces it?”

Unions have demanded assurances. The RMT said there was “zero chance” of agreeing to increase services until there was better protection for rail workers. A survey of its members found one in three felt poorly protected and had not been issued with PPE. A spokesman said: “We have the right to advise people not to work, if a workplace is seen as dangerous.”

Mick Whelan, the general secretary of the drivers’ union Aslef, said restoring services by 18 May would be unlikely and there “would need to be a very strong case made before we would be assured that it would be 100% safe to do so”.

The TSSA general secretary, Manuel Cortes, said he was “extremely concerned that briefings have been given to staff indicating a major remobilisation without fundamental questions being answered over a safe and sustainable plan.”

A Network Rail spokesperson said it was a decision for government, but it was “working closely on potential scenarios for an easing of the lockdown restrictions” and developing plans to reintroduce services “so that we are ready when required to act”.

The government has not confirmed or denied the May target, although the foreign secretary, Dominic Raab, said on Wednesday it would be “reliant on the data … in a couple of weeks’ time in order to even think about the next phase”.

A Department for Transport spokesperson said: “Any changes to service levels will be clearly communicated, subject to Public Health England guidance and discussed with industry and unions.”