Sir Vince Cable has criticised government reforms of trade union laws as provocative, unnecessary, vindictive and ideologically driven.
The former Business Secretary warned the plans could harm industrial relations and encroach on basic rights.
The Trade Union Bill, which will make it tougher for unions to call strikes, is expected to receive its second reading in parliament next week.
The Government says the legislation will make strike laws fair for workers.
The wide-ranging bill will impose higher voting thresholds in strike ballots, end the ban on the use of agency workers during strikes and introduce restrictions on picketing and protesting.
But Sir Vince said the moves were unnecessary and not based on evidence.
"I think it's ideologically driven and I think it's potentially vindictive and potentially counterproductive," he said.
"Most of the proposals that are now surfacing, we did look at very carefully in government and actually rejected them. When you look at the facts, only one in five ballots lead to strikes anyway."
The coalition government was at odds with unions over many issues, including the reform of public sector pensions and the privatisation of Royal Mail.
But Sir Vince said he found unions to be constructive, singling out the role of the Unite union in helping to secure the future of Vauxhall's Ellesmere Port plant.
"I think the trade union bill is not just unnecessary and vindictive, I think it could do harm. There are many respects in which trade unions work constructively and actively and this legislation puts that at risk."
And he warned that requiring more people to vote for strike action could backfire.
"Once you start imposing arbitrary and quite demanding ceilings, you make it more likely that if there is a strike, that it will be bitter and entrenched."
'Biggest attack in 30 years'
Frances O'Grady, general secretary of the TUC, said the bill went beyond the reforms brought in by the Thatcher government in the 1980s.
"This is the biggest attack on trade unions in 30 years," she said.
"This bill will poison industrial relations in this country and that's why I know there are many employers who are as worried about it as unions are."
She pointed to a new poll for the TUC indicating that the public thought new restrictions on trade unions would be a waste of police time.
"The public want the police to be out catching criminals" she said, "not wasting valuable time and resources supervising lawful picket lines and social media accounts".
This week, the legislation was criticised by human rights groups as "a major attack on civil liberties in the UK".
Liberty, Amnesty International and the British Institute of Human Rights said in a joint statement that the bill "would hamper people's basic rights to protest and shift even more power from the employee to the employer".
They also raised concerns over plans to restrict picketing and monitor the use of social media during strikes.
The Chartered Institute for Personnel and Development also described the bill as an "outdated response to the challenges of the modern workplace".
It highlighted the fall in days lost to industrial action.
Sir Vince said the Government was pushing through radical changes while the opposition parties were disorganised and unprepared.
"I worry now that the Tories are off the leash they can purse their ideological agenda and will do a lot of harm," he said.
"They are very political. They see the trade unions and the Labour Party as the enemy and the question is how do you weaken them? That is the starting point."
A Department for Business spokesperson said: "People have the right to know that the services on which they and their families rely will not be disrupted at short notice by strikes supported by a small proportion of union members.
"The ability to strike is important, but it is only fair that there should be a balance between the interests of union members and the needs of people who depend on their services."
Follow John on Twitter @JohnMoylanBBC