A Commons campaign to "bring trains on the Great Western line into the 21st century" has been launched by Bristol West MP Stephen Williams.
Liberal Democrat Stephen Williams claims it is reasonable to expect better services now than those that ran through the city when the 125 train was launched almost four decades ago.
While costs have spiralled, journey times between Bristol and London have got longer, he added.
Earlier this year the Government confirmed it was pushing ahead with plans to build a third runway at Heathrow, despite widespread opposition.
As part of a package of transport initiatives linked to the development, the Department for Transport is looking at options for improvements on links to the South West.
They include a new generation of "super express" trains, some of which are to be maintained at a newly-constructed depot in Stoke Gifford.
Last month the Government announced that it had awarded the £7.5-billion contract for the new trains to the Agility Trains consortium, with first services set to be running between Bristol and London by 2016.
And the project is expected to go hand-in-hand with the £100m electrification of the Great Western Main Line.
The Government is consulting on the electrification scheme and is expected to make an announcement on it later this year.
Mr Williams called for a quick decision to be taken on the electrification issue and said that the improvement of the Great Western Main Line is badly needed.
He said: "There are two main reasons why I have taken this move. Within 48 hours of the government announcement on the third runway at Heathrow I received more than 100 letters complaining about the plans.
"While the Government has made clear its timetable for the third runway it has not made clear its plans for electrification of the Great Western line.
"There is no clear commitment to electrification.
"I remember when the 125 service was launched. Since then journeys between Bristol and London have actually got slower."
In it he states that "the Great Western main railway line uses diesel trains originally brought into service in the 1970s and designed to travel at 125mph and in the 21st century modern rolling stock travelling at higher speeds than those envisaged 40 years ago is a reasonable expectation for passengers".
