I was on the London Underground last week when a woman got on at Archway station. She walked through the carriage, placing packets of tissues on the seats between passengers, with a note that read: “I’m a homeless mother of two children and need to support myself.”

My reactions were, in the following order:

1) You’re probably Romanian.

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2) You probably stole those tissues.

3) You probably don’t have children.

4) If I wanted tissues, I’d go to Boots.

My first reaction was: "My god. Am I racist?" But to the best of my knowledge, I’ve never uttered a racist remark in my life. Perish the thought.

However, later that day, as I blew my nose on a silky Kleenex (50p is a bargain to avoid an awkward silence), I thought of the times my mouth would have disowned my mind if it knew what it was thinking.

According to a recent British Social Attitudes Survey, one in three of us admits to being racist on some level. It’s fair to assume my tissue issues place me squarely in that shameful third.

I don’t doubt one in three of my fellow Tube passengers – including the “Essex chav”, “white van man” “estate agent” and “teenage mum” (we shameful third always subconsciously categorise everyone) – were thinking the very same thing.

My train shame throws into focus where the battle lines against bigotry are really drawn. Certainly not against the “boo hiss!” goons of the BNP. Those pantomime baddies are a clear and present danger.

Far more worrying is the unspoken prejudice that informs our interactions with each other. Those daily knee-jerk thoughts that make us judge others, without questioning ourselves for thinking them.

We are experts at sweeping our gut reactions under the carpet. But they still register loud and clear between our ears. What did you really think when Nigel Farage said he’d feel “uncomfortable” if Romanians moved in next door?

Or read in the Daily Mail that immigrants have cost Britain £140billion? Or watched yet another news report on immigrants putting a strain on housing, schools and jobs?

Did you find yourself wondering why spongers who can’t make a living in their own poor country come over here to live off the contributions of hardworking British people?

Why wouldn’t you? After all, the media promotes the libel that immigrants are only here for our taxes. Heck, there’s is even a term for it: “Benefits tourism”.

It’s not just immigrants. Any sub-culture outside the homogenous group we perceive ourselves to be a paid up member of is fair game for our subconscious.

The more you hear the classic Avenue Q song "Everyone’s A Little Bit Racist" the more it sounds as profound as anything Martin Luther King had to say.

Of course, occasionally our mouths reveal our mindsets.

Jeremy Clarkson again proved himself to be someone to avoid sitting next to at a dinner party with his “Eeny, meeny, miny, moe” apparent use of the ‘N-word’ while filming Top Gear.

One Direction’s Louis Tomlinson was recently caught appearing to use an abbreviated version of the ‘N-word’.

And footage emerged last week of Justin Bieber singing an ‘N-word’ parody of his hit One Less Lonely Girl – the second racially charged video of the singer to emerge.

Judging people is a vital survival skill taught to children. But the gap between judgment and prejudice can be treacherous to navigate. Britain in 2014 is a tolerant place to be. But the Social Attitudes Survey should serve as a wake-up call to us all.

Latent, if not blatant, prejudice bubbles beneath the surface of modern Britain. If we remain blind to our worst instincts and their consequences, we lose the battle at precisely the point where it needs to be won.

The next step is to change our thinking long before the words start spilling out our mouths.