David Cameron leads a government with a “worryingly authoritarian streak” that is seeking to undermine the Labour party’s funding arrangements through the trade union bill, a former head of the civil service has said.

Writing in a Guardian article, Bob Kerslake made one of the strongest attacks on a government by a retired senior civil servant, saying the bill marked a “partisan and disproportionate” attempt to improve the position of the Conservatives at the expense of Labour.

A confidential Labour party document released to the Guardian revealed that the party faced an expected £6m drop in its annual income as a result of the changes to the political levy being introduced in the trade union bill, making it impossible for it to maintain its current structure, staffing or offices.

Kerslake, who was head of the civil service between 2011 and 2014, said the bill and government reforms to the Freedom of Information Act were worrying developments.

“While there are quite reasonable steps [in the trade union bill] to increase transparency, the main thrust of the measures seems to be both partisan and disproportionate to the supposed problems that they are seeking to address,” wrote Kerslake, who was made a life peer in March.

“Taken with the other measures being put forward by the government – curtailing the powers of the Lords, watering down the Freedom of Information Act, cutting the so-called ‘short money’ to support the opposition parties – they demonstrate a worryingly authoritarian streak in this government that is not comfortable with scrutiny and challenge.”

Speaking in the House of Lords during the second reading of the trade union bill on Monday, Labour’s Lord Mendelsohn, who is a shadow business minister, said the proposed legislation was a “clear attempt to defenestrate the Labour party’s finances”, describing it as “extraordinarily partisan, vindictive and selective”.

He said Labour peers would do all they could to limit the bill’s effects. “We will not recoil from vigorously opposing that which is wicked and unjust, and we won’t shy away from suggesting improvements and suggestions. I have never thought that putting lipstick on a pig was a very good idea. But this bill has made me revise my opinion.”

In his maiden speech in the Lords, Labour’s former election strategy chief Spencer Livermore said it was a “curious anomaly” that the bill would leave trade unions as the “only organisation in Britain prohibited from harnessing technology to modernise their own democratic procedures”.