The Labour MP has held the Crosby seat since 1997
A Labour MP is to stand down at the next general election because of the "ludicrous" hours worked by parliamentarians.
Claire Curtis-Thomas won her Crosby seat with a huge swing in 1997 to become the area's first Labour MP.
The seat, which is to be renamed to be Sefton Central, has been identified as a key target for the Conservatives.
In a letter to the prime minister, Ms Curtis-Thomas cited family reasons for her decision to resign.
She said it was with "great regret and sadness" she had reached the decision, but that she remained committed to the Labour Party.
'Moribund male MPs'
"There comes a time when you have to think hard about your commitments, and the time you spend away from your family and the difficulties, stresses and strains that ensue," she said.
Ms Curtis-Thomas blamed a "collection of moribund male MPs", who failed to understand the need to balance family and work life, for the fact "parliamentarians still work ludicrous hours".
She then added: "I am no longer happy to walk the streets home at 12 o'clock at night having finished work an hour or so earlier. It is neither sensible nor practical.
"Relinquishing your role as an MP is never an easy decision to make, the Labour Party and our aspirations have been something that I have lived with since the day I was born."
Ms Curtis-Thomas also pledged her commitment to the party and declared her intention to campaign during the next election.
"There are difficult and unforeseen days ahead for us, and I trust you will do the best you possibly can for our future, our country and our people; we know this is an election that we must win," she added.
In 2004, Ms Curtis-Thomas topped the first list of MPs' published expenses having claimed £168,889.