Managing director Tom Clift told the Mail recently he expected a full timetable of three services a day between Hartlepool and London to be running again by the start of July.

But Grand Central has now admitted that engineering work to the trains' power cars is still being carried out.

Although Grand Central says it is confident of being able to provide a full timetable again soon, it will not give an exact date saying it does not want to let down passengers again.

The company has been reduced to operating just one train to the capital a day in each direction because of engineering problems with its used rolling stock.

A spokesman for the company said: "Significant works are continuing on a number of power cars and we believe we have got to the bottom of most of the engineering problems. We are optimistic that we can return to the full timetable soon."

Passengers have been plagued by delays and cancellations since Grand Central launched in Hartlepool six months ago.

After winning the right to provide direct services to London the operator refurbished old rolling stock instead of buying new trains.

But only two of its six power cars, placed at either end of the trains to help them reach top speeds, are in action.

The spokesman added: "We are keen to get back to a full timetable but we want to be comfortable that we can sustain that and don't want to let down our customers in Hartlepool."

He said a date for full services will be announced to passengers as soon as it is known.

Once the repair works have been completed Grand Central says it will carry out a period of testing to make sure there are no outstanding problems.