A proposed law to make it illegal for politicians to lie during Welsh elections contains “extraordinary” powers that would limit free speech, Senedd members have warned.

The bill will be debated in the Senedd on Tuesday but members, including those in Labour, have said it could fail to get enough support without significant changes like defining what a “false or misleading” statement would be.

The Welsh Labour government want make it a criminal offence to make false or misleading statements of fact to help an election candidate.

In a statement it said the legislation “sets out to build stronger foundations for Welsh democracy”.

Ministers had vowed to take action against lying among politics after a similar proposal almost became law against the Welsh government’s wishes last year.

Supporters of a ban on lying have argued that it could help rebuild trust and confidence in political institutions.

The ban would not be ready for the next election in May and would not come into force until the 2030 election at the earliest – although there is no timescale and one minister has suggested that even that date may not be achievable.

The bill has prompted two damning reports from two cross-party Senedd committees, with the legislation committee warning that the way it was written risked allowing a future government to “seriously hinder full and proper democratic discourse during an election campaign”.

It criticised the fact that it does not define exactly what a “false or misleading” statement is, to who it would apply or how.

Instead, it requires Welsh ministers to create a criminal offence only after the bill becomes law.

A report by a committee set up to scrutinise the bill warned ministers that if they failed to amend the provisions on lying it may not get the support it needs from Senedd politicians.

The committee, led by Labour Senedd member David Rees, said that “a power to limit free speech before or during an election with so few parameters as to the scope of any prohibition would be extraordinary as drafted”.

It added it was concerned the Welsh government was asking the Senedd “to give its endorsement to the creation of a new serious criminal offence which is undefined and could have life-defining repercussions”.

A majority of the committee felt that if it the bill could not be amended in time for the next stage of consideration in the Senedd, the section on false statements should be removed from the proposed law.

The committee also complained about the limited time it had to look at the bill, introduced late into the Senedd term, reducing the opportunity to speak to the public and others.

Rees said: “Strengthening the ways we hold members to account is essential for public trust, and the Senedd should get this right.

“While the committee supports the ideas behind the bill, it is clear to us that it will need significant amendment and real improvement if it is to be good law that will deliver on its aims for the people of Wales.”

BBC Wales was told Labour members on the committee and the Conservatives’ Sam Rowlands were among those who did not feel the government would have the time to make the necessary amendments.

Prof Jeremy Horder, a criminal law expert at London School of Economics, said he feared an “over-broad false statement law” could “have an unduly chilling effect on the speech of conscientious people and media outlets”.

The Welsh government’s most senior law officer, Julie James, said in response to the bill committee that it was “imperative that a full assessments of the impacts on the justice system… is undertaken”.

A Welsh government spokesperson said the legislation “sets out to build stronger foundations for Welsh democracy”.

“Having carefully considered the committees’ reports we have accepted, accepted in principle, or noted the significant majority of the recommendations ahead of the general principles debate,” they said.

Plans to allow Senedd members who misbehave to lose their seats through a ‘recall’ ballot of constituents are also a part of the bill, which will face its first vote on Tuesday.

The draft law does not legislate for a ban on Senedd politicians lying more generally, after a committee recommended that the existing regime of investigations by a parliamentary standards commissioner be strengthened instead.

It is already illegal to make a false statement concerning the personal character or conduct of a candidate during an election.