Salman Shaheen: Left Unity targets disaffected Labour voters
28 March 2014 Last updated at 14:54 GMT
The Left Unity group is picking up a new member every 10 minutes, said a member of its national co-ordinating group.
Salman Shaheen said it was appealing to "disaffected Labour" supporters, but it was "not a stitch-up of far left groups".
Andrew Neil asked about some of the motions going to its conference this weekend, and heard the group would be standing in elections, but was not ready to contest seats at May's European polls.
The late actor Roger Lloyd Pack was a supporter of Left Unity, founded by film director Ken Loach last year aiming to be a party for the working class.