He says there has been a "breakdown in trust" between the Scottish and UK Government and wants Labour, the Liberal Democrats and the Greens round the table to breathe new life into talks which have so far failed to reach an agreement.
Mr Leonard has now written to Cabinet Office Minister David Lidington and Scottish Brexit minister Michael Russell proposing a cross-party summit.
"As there appears to be a breakdown in trust and confidence between the two Governments there really needs to be an alternative way found forward," Mr Leonard told BBC Scotland's Sunday Politics Show.
"There is a lot at stake with the EU Withdrawal Bill and the devolution aspect of it.
"It appears that we are heading to a situation where the Scottish Parliament on Tuesday will almost certainly vote down a legislative consent motion on the Withdrawal Bill because of the treatment of devolution in it as it's currently constructed.
"I think there is still an opportunity to find a constructive way forward."
The concerns centre on on 24 key powers, in areas including farming and fishing, which will go to Westminster when they should rest at Holyrood in line with the devolution settlement.
UK ministers say these are to needed to ensure the integrity of the UK single market and ensure smooth UK-wide frameworks in areas like food standards.
A deal which offers a seven-year "sunset clause" has already been accepted by the Welsh Government which has similar "power grab" concerns.