SNP MSP: 'Independent Scotland should stay out of EU to avoid hard border with England'
He called for the party to advocate joining the European Free Trade Association (EFTA) instead of full membership, in a bid to ensure there is no hard border between Scotland and England.
Writing in The Herald, Mr Neil said the prospect of such a border would be detrimental to the independence campaign.
He also claimed it would not be possible to finalise Scotland's future as an independent nation in Europe until the rest of the UK confirms its relationship with the bloc.
The comments came on the same day as official Scottish Government statistics revealed Scotland's main trading partner remains the rest of the UK, with exports valued at an estimated £51.2 billion in 2018.
Mr Neil wrote: "If an independent Scotland joins the EU, that could lead to a hard border between Scotland and England, a proposition that would scupper any realistic chance of winning a second independence referendum.
"Until the UK-EU free trade agreement, or no agreement, is known it is not possible to finalise a policy.
"However contingency planning is sensible, and one possibility stands out - Scotland joining the European Free Trade Association (EFTA)."
Members of EFTA - Iceland, Lichtenstein, Norway and Switzerland - operate within the European single market but are not considered part of the customs union, meaning there would not be a need for checks on the border between Scotland and England.
EFTA members are also in the Schengen Area, meaning free movement is preserved.
Pledging to rejoin the EU means "handing a gift" to supporters of the union, according to Mr Neil, and would leave the case for independence "substantially weakened".
He added: "They would weaponise the border issue in exactly the same way they did with the currency issue in the 2014 independence referendum.
"The chances of winning the referendum would be unnecessarily and substantially weakened.
"These issues do not arise if an independent Scotland is part of a free trade agreement but not in a customs union. It is time to consider our non-EU options."