LONDON Underground PPP contractor Tube Lines is clashing with the train drivers' union Aslef over the legality of strikes which have been called over the next two weekends. Tube Lines is about to leave the private sector, after Transport for London announced that it would be acquiring the company's shares following a budget crisis.

The strikes are set to be staged in a dispute over pay, and would affect the running iof engineering trains. Their loss would disrupt planned engineering work, although public train services would not be involved.

Tube Lines is challenging tne legality of the strikes, saying that there were 'concerns' over the validity of the strike ballot. The company has urged Aslef to withdraw it. However, the union said the strike 'still stands', although further talks are taking place between the two sides.

Tube Lines itself is about to undergo a major change, and is set to follow its fellow PPP contractor Metronet into public ownership, following a TfL decision to buy Tube Lines shares. This move follows a major budget crisis, in which the PPP Arbiter had identified a funding gap of more than £400 million. The shortfall would occur in the next Tube Lines control period, which starts in July. The company is responsible for maintaining and renewing the Jubilee, Northern and Piccadilly Lines.

Metronet, which was responsible for all other Underground lines, collapsed in mid-2007 when the PPP Arbiter rejected most of its call for additional funding, and is now an in-house department of Transport for London.