London Midland has had to cancel about 40 trains because of the second 24-hour walkout in four days, in a row over Sunday working conditions.

The Rail Maritime and Transport (RMT) union said there was solid support for its strike action.

It said replacing strikers with managers led to "safety lapses". London Midland has denied this.

London Midland said services between London Euston, Tring, Milton Keynes and Northampton, and the lines between

Bletchley and Bedford, and between Watford Junction and St Albans Abbey, would be affected by the strike.

Around 40 of the 200 scheduled services between London and Northampton will not be running over the course of the day, London Midland said. Services to and from Birmingham were not affected.

Friday's industrial action involved about 100 staff.

The firm's managing director Steve Banaghan said using management staff in conductor roles during the industrial action had been approved by Her Majesty's Inspectorate of Railways.

"Many of our managers can draw on years of experience in conductor roles and they're doing an excellent job providing services to passengers during this period of industrial action."

Meanwhile, the RMT said it was going to ballot almost 100 revenue protection officers working on Southern Railway because of lone working.

The ballot, for action short of a strike, will open on 24 March and close on 7 April.

The union says having revenue officers working alone rather than in pairs raised the risk of them being assaulted.

In addition, voting in strike ballots across a number of train companies in a protest over job losses and changes to working conditions is due to close on Tuesday.

The train companies affected are South West Trains, National Express East Anglia, First Capital Connect (Thameslink) and London Overground.