One of the announcements at Nick Clegg’s press conference on Monday was his categorical rejection of Tory ministers’ attempts to abolish check-off across government.

What’s check-off? This isn’t a Michael Gove attempt to ban a celebrated Russian playwright, it’s a mundane administrative convenience which allows members of trade unions to pay their union subscription by getting their employer to deduct it from their wage packet and send it on for them.

Union members like check-off because it’s the easiest way to keep their subs up to date. Trade unions like it because it’s easier and cheaper for them to collect – savings which are in the members’ interests as they are reinvested in the union’s services. And employers don’t mind it either because if they want to they can charge the union the administrative cost of doing it so it costs them nothing, and it also shows goodwill – that they recognise the value of having a working relationship with a trade union.

Over the decades check-off has been the main method of collecting subs in the public sector, and it is widely used in the private sector as well. So the Tories, with their union-busting agenda, have been attacking check-off since Thatcher’s day. If they abolish check-off, the argument runs, then perhaps hundreds of thousands of members would be lost in the bureaucratic process of a mass conversion to other forms of payment. And that way they could undermine the trade union movement.

Under Thatcher they passed a law decreeing that every single union member who paid their subs this way had to sign a form, every three years, agreeing to continue with it. If they failed to do so and didn’t adopt an alternative method then they would lose their union membership. If the principle of this was so important why didn’t the Tories make companies do the same for everyone with direct debits? But the unions responded by organising highly efficient mass campaigns to sign up all their members, and the Tory plan failed.

This time round Frances Maude, Eric Pickles  and co decided on a full frontal attack. PCS, the main union in Pickles’s Department for Communities and Local Government, were simply given notice that the department was going to stop providing check-off. It would not have been possible in such a short time to switch all the members onto another payment option.

PCS responded by challenging the Department in court – and they won. During the case the court was told that the administrative ‘burden’ of checkoff costs the grand total of about £350 a year. But Pickles ended up costing the taxpayer about £90,000 in legal fees after he lost.

Undeterred, the Tories decided to try abolishing check-off across government. I understand that the plan was that they would put the proposal forward and had to get it agreed by either the Liberal Democrat minister in that department or Nick Clegg.

But Social Liberal Forum was alerted to the Tory attack by PCS and briefed Simon Hughes, then the Deputy Leader of the party. Hughes raised this issue with his colleagues in the party leadership and it was clear to all that the attack on check-off was, as stated on Monday, nothing less than a vindictive Tory attack on the unions.

The Parliamentary Party then took a collective decision to oppose this attack: Liberal Democrats believe that trade unions play a vital role in representing their members at work, in both the public and private sector.

Congratulations to Simon Hughes, Nick Clegg, Danny Alexander and Vince Cable for their principled decision which has ensured that the workforce in government continue to have the union protection that every worker needs.

* Janice Turner is Vice Chair of the Liberal Democrat Campaign for Racial Equality