Holyrood's rural economy committee has refused to back a bill seeking to make 20mph the standard speed limit on residential streets in Scotland.
Green MSP Mark Ruskell put forward the legislation with the aim of reducing deaths and serious injuries on roads.
But a majority of MSPs on the committee said the "one size fits all" approach was not appropriate and would not give local authorities enough flexibility.
Three members backed the move, calling the current system "confusing".
And Mr Ruskell said the committee had "put the motoring lobby ahead of child safety", and urged SNP members to "find a backbone" before the bill is voted on in the chamber in June.
He put the bill forward for scrutiny at Holyrood after winning the backing of 25 Green, SNP, Labour and Lib Dem MSPs, as well as environmental groups and round-the-world cyclist Mark Beaumont.
It proposes making 20mph the standard speed limit for "restricted roads" - chiefly residential and minor roads in urban and rural areas - although it would give councils the power to designate 30mph areas in consultation with communities.
Councils can currently create 20mph zones - this has happened in Edinburgh, Fife and Clackmannanshire - but campaigners say this is "time consuming and costly" to do, and creates a confusing "patchwork" system.
Mr Ruskell said reducing the default speed limit was "the simplest, quickest and cheapest way to save lives, make our streets safer, and encourage more people to walk and cycle more".
The majority of members on the rural economy committee did not agree, saying they were unable to recommend the bill to parliament.
Their report voiced backing for widening the rollout of 20mph zones in Scotland, but questioned whether Mr Ruskell's legislation was the best way of doing it.
It reads: "After considering the evidence presented, the majority view of the committee is that the default, 'one-size-fits all' approach proposed in the Bill is not appropriate, as it does not give local authorities the flexibility to devise 20mph limits that they consider appropriate for their areas."
The group also said the financial plans underpinning the bill were "not robust".
However, they did say that the existing process by which councils can establish 20mph zones was "cumbersome and resource-intensive" and "should be more straightforward".
'Find a backbone'
The report was endorsed by eight members of the committee, including SNP, Conservative and Lib Dem MSPs.
But three - Green John Finnie, Labour's Colin Smyth and SNP member John Mason - dissented, saying that the "current inconsistent use of 20mph speed limits" was "confusing and undermines road safety".
Mr Ruskell said the report "puts the motoring lobby ahead of child safety", urging SNP members to get behind the bill when it comes to a vote.
He said: "I know that there is disquiet about this among some SNP backbenchers, and I hope that the first minister will listen to them and the local councils, health, children's and environmental organisations that are backing the bill.
"My bill will be put to a vote in two weeks time. That's how long the SNP have to find a backbone and get behind it. They claim to be the party that embraces change, yet they've teamed up with the Tories to block progress."