The trade union for train drivers, ASLEF, announced it is to stage walkouts on Wednesday 26 July and Friday 28 July.

Strikes had already been announced on the transport network by the RMT union, representing different grades and sections of Tube staff, on Sunday 23 July and daily from Tuesday 25 to Friday 28 July.

Read more:
Full list of July train strike dates

Tube drivers are taking the action in a dispute over pensions and working condition changes, ASLEF said.

"Unfortunately, the last few weeks have shown that London Underground management are determined to try to push through detrimental changes," ASLEF official Finn Brennan said.

"To protect our pensions, working conditions, and agreements, our members are ready to act."

The union said managers will begin training in August with a new attendance procedure meaning its members will have no right to representation or appeal in the beginnings of a disciplinary process, and the length of disciplinary warnings would be doubled from 26 to 52 weeks.

It added that Transport for London (TfL), Tube workers' employer, is changing sickness practices, classifying anything longer than a week as long-term.

"Their aim is an entirely flexible workforce with all existing agreements replaced - allowing them to cut hundreds more jobs and forcing those of us who remain to work harder for longer," Mr Brennan said.

TfL's chief operating officer, Glynn Barton, said, "We are disappointed that ASLEF has announced strike action which will coincide with the action already announced by the RMT.

"We are urging both unions to reconsider this action and engage with us to discuss the issues and seek a resolution.

The employer denied it was changing pensions.

"There are no current proposals to change pension arrangements and, although we are discussing with union colleagues a range of proposals to improve how London Underground operates, no employee will lose their job or be asked to work additional hours," Mr Barton said.

"We are trying to create a fairer, more efficient London Underground that works for our staff and for London.

"All stations would remain well staffed at all times and we believe that our proposals would give an even better and more reliable service to our customers both in terms of our stations and our train services."