The service runs from the East Midlands to London

There will only be one hourly train from St Pancras, Nottingham and Derby, with two from Leicester. A limited Derby to Sheffield shuttle will run.

The RMT union said Midland Mainline (MML) is compromising on safety by not putting two guards on each train.

But MML said a risk assessment was carried out by an independent company, which found this would be safe.

It is operating a revised timetable on four consecutive Fridays - 27 May plus 3, 10 and 17 June.

'Sincerely apologise'

It is offering a full refund for people who prefer not to travel, or they can use their ticket on any train on Friday or Saturday, regardless of booking.

The Derby-based company said: "We understand that this may cause some disruption to those wishing to travel and for this we sincerely apologise."

The RMT is concerned about services which are in effect two trains locked together, with no access between the two sections.

The union said if there was an incident on one section of the train, the guard might not be able to reach it.

Guards have been working to rule and have banned rest day and Sunday working, but have now decided to step up their action.