Labour should explain why it took five months to discipline an AM convicted of a "serious crime", the ex-chairman of the Committee on Standards in Public Life Sir Alistair Graham has said.
Rhianon Passmore was convicted in February of failing to provide a breath test and referred herself to an assembly watchdog in March.
The offence happened in September 2017.
Welsh Labour has suspended the Islwyn AM from its assembly group but will not comment on the disciplinary process.
On publication of the assembly standards committee report on Monday - which recommended she be excluded from the Senedd for 14 days, the Labour assembly group said it had withdrawn the whip from Ms Passmore until 1 October.
Sir Alistair advised Tony Blair's Labour UK government on standards in public life between 2004 and 2007.
He criticised Ms Passmore's delay in informing the assembly's standards watchdog of her conviction at Newport magistrates court in February.
"I am surprised that she was rather slow in referring herself to the standards commissioner but she was clearly right to self-refer," he said.
"I am also rather surprised Labour didn't initiate a disciplinary process or investigation as soon as the assembly member was convicted of such a serious offence."
"They [Labour] have given no explanation about why they didn't take action until the standards committee reported on the issue."
Ms Passmore has apologised, referring to "private struggles that I have found difficult to master".
She said on the day of her arrest she had tried to drive from Cardiff to her mother's house after drinking three glasses of wine.
Her car broke down in the St Mellons area of the city - while she was waiting for the breakdown service, the police arrived and asked her to provide a sample of breath for analysis.
The former probation board member, elected in 2016, was given a 20 month driving ban, £1,000 fine and £100 victim surcharge while prosecution costs of £620 were awarded against her.
In written evidence to the commissioner, Ms Passmore said she did not deliberately fail to give a sample and was distressed after not being able to use her asthma medication properly.
Plaid Cymru AM Simon Thomas commented on the lack of publicity about the matter, and asked if her Islwyn constituents had been aware of it.
He said on Twitter: "Make of this what you will but I actually knew nothing of this until it appeared this week.
"If I didn't then I suspect most constituents did not, which is a bit strange for such an offence."