Lord Dunlop, a former Conservative minister for Scotland and Northern Ireland, labelled Boris Johnson’s style of government “shambolic”.
The former minister previously worked under both the Cameron and May administrations and was key in the battle to win the independence referendum in 2014.
However, speaking to the Sunday Times, he said that the Prime Minister does not have the skill required to hold the union together.
Nicola Sturgeon aims to pass legislation enabling a second independence referendum by the end 2023, however the SNP leader is likely to require the acquiescence of any Westminster government for the referendum to go ahead.
Mr Johnson is facing calls to step down as Prime Minister due to the ongoing Partygate scandal, with leaks and photos showing evidence of widespread lockdown rule-breaking inside Downing Street.
Lord Dunlop said: “He [Boris Johnson] is another accident waiting to happen. That’s why his position needs to be resolved sooner rather than later. Otherwise his shambolic governing style will continue to sabotage the substance of the government’s work.”
“There’s no more important work than strengthening our UK Union. This week the government announced an agreement with the devolved administrations brokered by Michael Gove on important reforms to how they work together more effectively. Their success will depend on the leadership, commitment and attitude of the prime minister.
“I don’t see Boris, whose authority is so weakened, being able to provide the necessary leadership the country needs at this time. It pains me to say it, but it’s time for him to go.”
The comments follow the decision from Scottish Conservative leader, Douglas Ross, to demand Mr Johnson’s resignation earlier this week and write a letter of no confidence to the backbench 1922 committee.
Despite being supported by all 31 of the Scottish Tory MSPs in Holyrood, cabinet minister Jacob Rees-Mogg labelled Mr Ross a “lightweight” politician as the schism between the UK and Scottish Tory party grew.
Sue Gray, who is investigated the many parties in Downing Street, is likely to report back by the end of this week.