In direct contrast to the stance of Scottish Conservative leader Douglas Ross, Mr Tomkins believes the Supreme Court could decide a referendum Bill approved in Holyrood is lawful as it would only be seeking the opinion of the Scottish people.
Nicola Sturgeon has said she will push for a second independence referendum before the end of 2023, but would seek to hold one through agreement with the UK government, as happened in 2014.
She has also suggested however, that if Prime Minister Boris Johnson continues to refuse to grant a Section 30 Order, the Scottish Government could hold its own referendum and it would be up to the UK government to challenge it in court.
During the election campaign Douglas Ross said any poll organised without Westminster consent would be a “wildcat referendum” and not be valid.
However speaking to the Daily Record, Mr Tomkins, who stood down from Holyrood in May, said while there was an “assumption” the Supreme Court would strike down a Holyrood law on a referendum, he did not think the law “is as clear as that.”
“Over the course of the last of the five years, the Supreme Court has given a number of judgements that I think have very significantly muddied the waters,” he said.
“If the Scottish Government presents an independence referendum bill as if its purpose is to seek the opinion of the Scottish people, knowing that its effect in law is zero, then because of the Supreme Court’s judgements since Brexit, it is going to be quite difficult I think to convince the Supreme Court that this a measure that relates to a reserved matter.”
He added: “The effect of the measure might be politically enormous, but the legal effect of the measure is zero.”
Mr Tomkins’ remarks were welcomed by SNP MSP Rona Mackay, who urged him to “use his expertise to convince Boris Johnson and Douglas Ross to respect democracy rather than continuing their Trump-like bid to deny a clear-cut election result.”
A Scots Tory spokesman said: “The last thing that Scotland needs right now, with a health and economic crisis to tackle, is another divisive independence referendum."