Tamsin Omond doesn't live like the granddaughter of a baronet, a candidate for priesthood or a first-class honours graduate from Cambridge who will be sitting her Open University MA examination tomorrow. Despite a flat crammed with political treatises and teetering heaps of campaign banners, high-visibility protest jackets and demonstration handouts, Omond is all three.

In a few hours' time, however, the dissonance between Omond's lineage and her surroundings could be even greater. Tomorrow, she could exchange her chaotic but comfortable north London flat for a single room provided at Her Majesty's pleasure, via an examination hall and a high-profile Westminster demonstration attended by the likes of Baroness Tonge and Rosie Boycott.

'I don't want to go to jail. I don't want to break the law,' said 23-year-old Omond. 'The idea of going to prison makes me sick and nervous. It's very definitely not part of my life plan and I absolutely don't want it to happen. But if that's the price of protest, I find myself in a very difficult position.'

Omond is the founder of Climate Rush, the group behind tomorrow's demonstration in Parliament Square that will call for an end to airport expansion and tougher measures to control carbon dioxide emissions. It is a case in which she passionately believes: last February she was one of the so-called 'Commons Five' whose faces flashed across news programmes throughout the world when they scaled the Houses of Parliament to protest against a third runway at Heathrow.

The group stayed on the roof for three hours: a period carefully timed to coincide with Gordon Brown's arrival for Prime Minister's questions, during which at least one MP was to ask questions about the anti-aviation debate. Brown cancelled at the last minute - Omond is not sure whether his non-appearance was linked to the protest - and the group descended from the rooftops to be met by police who arrested them on charges of trespass. The group were kept in custody for 12 hours before being released on bail to await their trial on 11 November, at which they could receive a maximum penalty of 51 weeks' custody and a £5,000 fine.

But if the idea of a year's incarceration makes her feel queasy, it is the bail conditions that are causing her immediate concern. 'If I go into the Houses of Parliament or break the law before the court case, I will be held on remand,' she admitted. 'Part of me doesn't want this life at all. But if you look back at history, the world has been changed by people with the courage to fight for the right thing at a time when that thing was being completely ignored. This is exactly the position we're in now.

'What I find most shattering is this amazing discrepancy between what is scientifically proven and what the public are being told,' she added. 'Time is fast running out to stop irreversible climate change. Global warming experts agree that we have only 100 months to avoid disaster, and yet we're in this ridiculous situation where the public are saying climate change can't be that bad, otherwise the government would do something. And the government is saying it is that bad, but we can't do something because the public's not ready for it.'

It was this Alice in Wonderland world that, five years ago, shunted Omond on to the path she is now hurtling down. 'I was in my final year at Cambridge and had my life planned out: I was going to go into marketing and get a nice job in the City. Then one day, instead of reading King Lear for the eighth time, I picked up some climate change research and found I couldn't put it down.'

In the year after graduating from Cambridge with a first-class honours degree in English, Omond continued her environmental research. 'I became completely convinced by the evidence that we only have a really limited time to make a large social change,' she said. 'I realised that direct action was the only way to raise public consciousness to a point where those changes could be made.'

Omond had never attended a protest but she took to grassroots campaigning like a duck to water; helping co-ordinate carnival marches, protest camps, lock-on's (attaching herself to buildings and gates with locks) and banner-drops. In December 2007, she became London co-ordinator for the environmental network Plane Stupid, organising protests at corporate offices, high-street travel agents and local airports.

Last July, she was one of the group who blasted aircraft noise through the letterbox of the then Transport Secretary Ruth Kelly's Docklands apartment at 7.30am - leaving peacefully, albeit pursued by Kelly's husband, Derek Gadd.

Shortly after Omond left Cambridge, she had decided she wanted to become a priest. Already a church administrator, she asked her vicar to initiate conversations with the local bishop about beginning her training. It is, she now admits, a dream she might never realise.

'I would love to return to training for the priesthood, but this whole period of me breaking the law might mean that they don't want me any more,' she said. 'I desperately want to be a priest, but I don't have the time to commit myself to the Church and to activism at the same time. I had to pick one and, at the moment, my environmental action is more important.'

Tomorrow afternoon, Omond will gather together her banners, handouts, high-visibility jackets - and her revision notes - and head off to sit her MA. It is a three-hour examination but she has to finish it in two hours if she is to reach Parliament Square by 5.30pm and catch the impressive list of speakers who support the cause: Rosie Boycott will be followed by, among others, Sam Roddick and Baroness Tonge.

The line-up concludes at 6.30pm with a speech from Omond herself. And then there will be an hour left to protest in the best way the group know.