Boris Johnson is whipping up fears of rioting and deaths on the streets if Brexit is not delivered by 31 October so that he can try to invoke emergency powers and avoid extending the UK’s EU membership beyond that date, Labour’s Brexit spokesman, Keir Starmer, claimed on Saturday.

After a week in which the prime minister was accused by MPs from all the main parties, including senior Tories, of inciting violence by accusing Remainers of Brexit “surrender” and “betrayal”, Starmer said it was part of an orchestrated plan to stoke a sense of outrage among Leave voters and create civil unrest, so an extension might be avoided.

Increasingly MPs across the House of Commons believe Downing Street is considering using the Civil Contingencies Act 2004, which grants special powers in the event of a national emergency, as a way to override the so-called Benn act, which mandates the prime minister to seek a delay to Brexit if no deal has been struck with Brussels by 19 October.

Johnson’s description of the act as the “surrender act” provoked heated exchanges in parliament last Wednesday. Female MPs invoked the memory of their murdered colleague Jo Cox and said they feared for their safety. on Saturday night the prime minister reinforced his rhetoric in a newspaper interview saying his “humdrum political metaphor” could just as well have been the “abject capitulation act”, and questioned whether the EU had been consulted during the bill’s drafting.

Starmer told the Observer that Downing Street’s strategy was clearly to stoke tensions but he warned Johnson that any attempt to invoke the Civil Contingencies Act would be defeated in the courts.

Shadow Brexit secretary Keir Starmer. Photograph: Gareth Fuller/PA

“Whipping up the idea of riots or even deaths if we do not leave the EU on 31 October is the height of irresponsibility,” Starmer said. “But it is also pretty obviously being orchestrated. If this is part of a government plan to misuse powers under emergency legislation, I can assure the prime minister we will defeat him in court and in parliament.”

Johnson is expected to continue his attacks on Remainers this week at the Tory conference in Manchester where the slogan will be “Get Brexit Done”.

Former Tory MP and attorney general Dominic Grieve said he could see no other possible way Downing Street could comply with the law and deliver Brexit on time. “The Civil Contingencies Act is the only possible route I can imagine they can be thinking of,” Grieve said. “But if they do try to do this it would be a constitutional outrage. And if it passed through parliament it would be immediately challenged in the courts.”

Allies of Johnson have warned in recent days that the UK will face civil disorder on the scale of the gilets jaunes protests in France if Brexit were to be delayed or reversed.

Senior Labour sources said that possible use of the Civil Contingencies Act to trump the Benn act was on a list of options that the party’s legal advisers believe Downing Street might use.

After 11 supreme court judges ruled unanimously last week that Johnson’s decision to shut parliament for five weeks had been unlawful, and declared the prorogation null and void, MPs say there are signs of increasing desperation at No 10 as it faces the prospects of more defeats in parliament and the courts.

On Saturday night, Johnson signalled that he would again challenge the judiciary by introducing US-style political screening of supreme court judges before they are appointed. Speaking to the Sunday Telegraph, the prime minister said: “If judges are to pronounce on political questions in this way, then there is at least an argument that there should be some form of accountability. The lessons of America are relevant.” The Law Society and the Bar Council both said last week that politically appointed judges would threaten the independence of the judiciary.

Some minor parties are pressing hard to pass a motion censuring Johnson, forcing him to apologise for the language he has used against his Brexit opponents. Plaid Cymru’s Liz Saville-Roberts has even asked Speaker John Bercow whether she can table a motion to impeach Johnson. Another avenue being examined is to censure Johnson should he appear before the liaison committee of MPs.

Last week the former Tory prime minister John Major said he feared the Johnson government would try to bypass the Benn Act through an order of council. “It is important to note that an order of council can be passed by privy councillors – that is government ministers – without involving HM the Queen,” he said, adding: “I should warn the prime minister that – if this route is taken – it will be in flagrant defiance of parliament and utterly disrespectful to the supreme court. It would be a piece of political chicanery that no one should ever forgive or forget.”

The Civil Contingencies Act provides the government with powers to create emergency regulations at times of national crisis and threats to safety (including wartime), emergencies that threaten “serious damage to human welfare”, or to the environment or the security of the UK. Damage to human welfare is defined in the act to include disruption to transport networks or to the supply of food, money, energy, or health services.

Downing St sources said it was not planning to use the Civil Contingencies Act. A spokesman added: “The prime minister is determined to deliver Brexit on time and he will abide by the law.”

Senior Labour sources played down suggestions by the Scottish National Party that the opposition parties could this week table a vote no confidence in Boris Johnson’s government, with a view to installing a caretaker administration led by Jeremy Corbyn who would then apply for an extension to Brexit and call an election.

While Labour is not publicly ruling out the move, shadow cabinet members privately say they will only move once an extension to Brexit has been guaranteed. The Liberal Democrats and other opposition parties, as well as former Tory MPs and independents, say they will not under any circumstances agree to install Corbyn in Downing Street, even temporarily.

Anna Soubry, leader of the Independent Group for Change, which has five MPs, said: “We need to keep the prime minister held firmly accountable for the mess he has created. We do not want any votes of no confidence until that extension is up and running and even then I will only support a vote of no confidence knowing that it will result in a government of national unity resulting in a people’s vote confirmatory referendum.

“Anything else gives Johnson his escape route. His only plan B is a general election and a general election would not solve Brexit. Only a referendum will sort out Brexit.”

On Saturday the Liberal Democrats announced that Dr Phillip Lee, who recently defected from the Tories, will be their candidate to fight the veteran Tory and hard-line Brexiter John Redwood in Wokingham, to set up a Leave v Remain contest in the Berkshire seat.