Bus giant Stagecoach has been accused of "underhand methods" and "dirty tactics" during a broadside delivered in the House of Commons.
Preston Labour MP Mark Hendrick criticised the firm for trying to push the Preston Bus Company off the road.
His attack, during business questions in the Commons, marks a new front in the Lancashire city's bitter bus war.
Addressing Leader of the Commons Harriet Harman, he said: "What we are seeing at the moment is a terrible situation where a bus company, Stagecoach, is using underhand methods and dirty tactics to try and push the Preston Bus Company off the road.
"I would be very grateful if she would look at this in some detail."
Ms Harman vowed to raise his concerns with her cabinet colleague, transport secretary Ruth Kelly.
She said: "I will draw the secretary of state for transport's attention to the points that he makes about important questions of public transport in Preston."
The feud between the two bus companies has already led to official warnings from government bosses, complaints from members of the public and even bouncers and watchmen on bus stops across the city.
It started in May when Stagecoach registered to run a service every 10 minutes from the city centre to Gamull Lane, Brookfield and Farringdon Park, via New Hall Lane.
The new service undercuts Preston Bus fares on two of its most lucrative routes.
Stagecoach stepped things up further by adding more money-spinning services to Royal Preston Hospital and Tanterton.
A Stagecoach spokesman said: "It is extremely disappointing that Mr Hendrick is using the cowardly cloak of Parliamentary privilege to make these completely false claims.
"We have invested more than 4million in improving bus services in the area, created new jobs for local people and attracted many people to our services who have previously never used the bus."
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