Labour expect to see a £6 million drop in funding due the effects the Tories’ Trade Union Bill, according to The Guardian.

The paper has obtained a confidential Labour Party document that calculates the financial effect of the anti-trade union legislation. The bill, which is being debated in the House of Lords today, will require an “opt-in” to the political levy for union members. It is expected that this will drastically reduce the number of trade unionists directly affiliated to the Labour Party, possibly by around three million.

Sources within the trade union movement have privately expressed concern that some figures within the Labour Party had yet to fully grasp the enormity of the funding gap that the bill would create. The emergence of this new document suggests that its effects can no longer be ignored.

The leaked report states that around 10% of trade unionists are likely to opt-in to support Labour financially, based on a review of trade union involvement in the 2010 leadership election. This would mean the number of members affiliated from the five biggest unions would fall from around 3.4 million to 330,000.

Estimates suggest trade union funding of Labour will fall from £7.9 million to £1.9 million, with a reduction in Short Money, also being pushed through by the Tories, from £6.5 million to £5 million over the course of the parliament.