The report by the House of Lords Select Committee on Trade Union Political Funds and Political Party Funding accepted that the bill would result in reducing the Labour party's funding, and as such made recommendations that would result in the government having to make concessions to Labour.
The committee agreed that new members should have to make an active choice about whether or not to opt-in to the fund, but added the transition period to an opt-in system for new members should be increased from three months to a minimum of 12 months.
"We have made a series of recommendations to the House which would allow the government to fulfil its manifesto commitments while mitigating the worst impacts on the trade unions and the Labour party," Lord Burns, chair of the committee, said.
"The committee failed to agree only on whether or not the government’s provisions should be extended to existing union members, and it will be for the House to make its mind up on that point," he added.
The report also proposed that trade union members should not have to renew a decision to opt-in every five years, instead having to opt-out.
The question of whether to extend the opt-in to existing union members must only be considered as part of the cross-party discussions on party funding reform, the report added, and should not be dealt with in this Bill. It also said the government should convene cross-party talks on party funding reform.
Labour leader Jeremy Corbyn has previously branded the government's Trade Union Bill as an attempt by the Conservative government to "tighten their grip on power".
However, commenting on the report, minister of state for employment law Nick Boles said: “The committee has accepted the principle that members should be asked to make an active choice about whether to opt-in to a union's political fund."