Against all the odds, union membership rose in Britain last year for the first time in a decade. According to the annual statistical report by the Department for Business, Innovation and Skills (BIS), membership for 2012 was 59,000 higher at 6.455 million workers compared with 2011.

Of course, this rise still means that union membership is below where it was prior to the beginning of the global financial crisis in 2007. Then it was 6.999 million and in 2002, a decade earlier, it stood at 7.020 million.

But given the scale of the recession in the private sector (especially in manufacturing, retail and finance) as well as the age of austerity in the public sector leading to huge job cuts and rising unemployment, even a small overall rise in union membership is a considerable achievement.

The breakdown of the new figures shows that membership grew in the private sector from 2.509 million in 2011 to 2.586 million in 2012, while in the public sector it steadied at 3.883 million in 2012 (down from 3.886 million in 2011). This meant that union density in the private sector increased from 14.2% to 14.4% while in the public sector it fell from 56.6% in 2011 to 56.3% in 2012. Overall, union density remained the same for 2012 as it was for 2011 at 26%.

The figures from BIS are consistent with those from the first findings from the latest Workplace Employment Relations Study. This makes the small recorded membership increase for 2012 far less likely to be a technical blip resulting from sampling errors.

So what accounts for this surprise rise in membership?

First is the unions' ability to show that they are the best available collective defence in times of need. Often unions are unable to practically do much to prevent redundancies when they have been announced but the fear of redundancy and job insecurity is motivating employees to join.

But even where the threat of redundancy is not so present, workers who remain in work fear the downward pressure on their terms and conditions of employment. For example, section 188 redundancies – a technical but legal means by which employers unilaterally impose changes in terms and conditions – are not uncommon. They are occurring in the third sector, in the public sector and in manufacturing.

The Unite union is a good example of a large union that has indicated it is now prepared to take a more hardline position with employers over such issues. The same could be said of the Public and Commercial Services union in the public sector.

Second, unions have put considerable resources into employing organisers and training activists to recruit and retain members. For example, in the public sector Unison's recent two-week spring campaign saw 63% more people joining in the first week compared with the same week last year and the campaign is being rolled out regionally now. This effort applies mainly to existing areas of union organisation (called "brownfield" sites) where the aim is to top up membership.

Third, and finally, more resources are being put into "greenfield' organising where membership has not existed before (either at all or in any significant way). The Unite union is among those leading the way here but smaller unions such as the broadcasting and entertainment union Bectu, are also doing their bit.

Whether the trend of membership stabilisation and limited growth continues remains to be seen – but at least a firmer foundation for it to take place on has been created.