Greens are unusual for a political party - though we're small, we're one of the few political parties in Britain in which membership has actually grown over the recent years - membership is around 40% up over the past year.

Over the years, I've had many friends who have been active members of the Labour Party.

Some of them - long-term members and people with deep principles - left Labour in despair over issues from the illegal invasion of Iraq to the erosion of civil liberties and the renewal of the Trident Nuclear weapons system.

It is interesting that possibly as much as half the Green Party in Brighton and Hove is made up of former Labour Party members.

However it's not easy to leave your political ‘tribe', especially if you've been a long-standing activist.

It's a bit like a football team you instinctively remain loyal to - ask Peter Tatchell who crossed after many years in Labour to the Greens.

I really admire those who have had the courage to do so.

Some ‘Old' Labour activists will claim that they're still in Labour to change the party from within and to haul it back to its founding principles.

But when I look at some Labour MPs whom I've admired over recent years: for example, Clare Short - left the party - and Alan Simpson - leaving Parliament - I see people who've tried bravely to influence the direction of their party but have failed.

If they failed, what chance do less experienced members have in the face of a monolithic party machine?

So I sometimes ask myself, "What would it take for me to decide to leave my ‘tribe'?"

Well....

*If it took us into an illegal war, I'd leave my ‘tribe'.

*If it committed £78 billion on building and running a replacement for Trident, I'd leave my ‘tribe'.

*If one of its leading policy-makers articulated the view that, "We are intensely relaxed about people getting filthy rich", I'd leave my ‘tribe'.

*If the actions of its leaders contravened my core principles, I'd leave my ‘tribe'.

Fortunately I don't think these are likely to be Green Party positions in the near future...

Last summer, my husband asked an old friend, a Labour activist for 33 years, "What would it take for you to leave the Labour Party?"

"I'm one of those people who tends to work on the ‘glass half-full' theory of life.  I focus on the positive," was his friend's reply.

"But, there comes a time," my husband responded, "when you have to consider what it is that you are drinking."

His friend has now left Labour and is supporting the Greens.