The change of heart, which follows two years of talks and feasibility studies initiated by South London Learning and Skills Council, was prompted by a Whitehall policy U-turn in favour of separate specialist colleges.
Christina Conroy, principal of Richmond adult community college, said that consultations with students and staff had shown that there were 'culture clashes' and irreconcilable differences between them and the teenage atmosphere at the tertiary college.
She added:"Recent research from Warwick university for the Department for Education and Skills and LSC has shown that mergers are not quick-fix solutions and can be destabilising. There are few short-term benefits except that of saving a failing college, which does not apply here."
Alan Johnson, minister for further and higher education who visited the college last week, had told college chairs to concentrate on their own distinctive mission. A meeting with the tertiary college governors before Christmas had been "convivial and positive".
Ms Conroy said: "We realised that it was not easy to reconcile our cultural differences, so we decided it is best to collaborate rather than merge."
But Eric Kirby, principal of Richmond upon Thames tertiary college, who came back from retirement to oversee the merger talks, was disappointed.
"We believe the merger would have benefited the community," he said. "As the lead FE college in London we will now continue to develop our post-16 provision and push forward plans for new accommodation and an extension of our courses'.
The merger talks had originally been encouraged by the Croydon-based LSC to rationalise courses and buildings between the two colleges.
Executive director Jay Mercer, a former LEA officer in Richmond, said: "The borough is fortunate to have two well-run colleges. It is unfortunate that the merger is not going forward. However, we will continue to work with both and maintain every effort to ensure they work collaboratively."
Meanwhile, the two colleges were this week competing for part-time students in adult and vocational courses for the spring.
Councillor Geoffrey Samuel, Tory cabinet member for education on Richmond upon Thames council, said: "Let competition thrive. It will drive up standards." He said he would be talking to both colleges about tendering to revive sixth forms in the borough's secondary schools as well.