British Prime Minister Boris Johnson's controversial plan to override key elements of the Brexit deal he signed with Brussels has cleared its first Commons hurdle despite deep misgivings by some senior Tories.

MPs voted to give the UK Internal Market Bill a second reading by 340 to 263 - a government majority of 77.

Mr Johnson said the legislation was necessary to prevent the EU taking an "extreme and unreasonable" interpretation of the provisions in the Withdrawal Agreement relating to Northern Ireland.

The Internal Market Bill will see the UK government reserve the right to unilaterally interpret the Northern Ireland Protocol's rules on state aid and customs declarations.

The EU's legal advice says the Northern Ireland Protocol "forms an integral part of the Withdrawal Agreement... If adopted as proposed, this bill will be in clear breach of substantive provisions of the Protocol…"

Mr Johnson said some in Brussels were now threatening to block UK agri-food exports to the EU and to insist on tariffs on all goods moving to Northern Ireland from the rest of the UK.

However some senior Conservatives warned they could not support the legislation in its present form after ministers admitted last week that it breached international law.

MPs will begin detailed line-by-line scrutiny of the Bill tomorrow, with votes expected next week on amendments to the Northern Ireland provisions which some Tories may back.

All of Britain's living former prime ministers have expressed concern about Mr Johnson's plan.

His previous finance minister, Sajid Javid, said he could not support the bill unless it was amended.

"Breaking international law is a step that should never be taken lightly," Mr Javid said in statement.

"Having carefully studied the UK Internal Market Bill it is not clear to me why it is necessary to do so."

Conservative Rehman Chishti, who was Mr Johnson's special envoy for freedom of religion, quit his role over the issue while Mr Johnson's former Attorney General Geoffrey Cox, who is influential with colleagues, was also critical.

"No British minister should solemnly undertake to observe treaty obligations with his fingers crossed behind his back," Mr Cox said in The Times newspaper.

The leader of the opposition Labour Party, Keir Starmer, who was self-isolating due to a possible coronavirus case in his household, said he would oppose a bill that broke international law.

Mr Johnson's plan to explicitly break international law has plunged Brexit back into crisis less than four months before Britain is finally due to leave the EU's orbit at the end of a transition period.

Earlier, Northern Ireland's First Minister warned the European Union to stop using the region as a "play thing".

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The Tánaiste said he hopes the UK's internal markets bill is just a benign negotiating tactic on behalf of the British government and will be unsuccessful. 

Leo Varadkar said if it was anything else, it would be a sad day for a country that was going to breach its international obligations and law.

He said there was considerable opposition to the bill and the British government would find it very difficult to get it through in any event.

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British ministers say the bill, which explicitly states that it could be inconsistent with a host of international laws, is intended to clarify ambiguities - particularly over Northern Ireland - and act as a safeguard in case trade talks fall.

But some EU diplomats believe London is playing a game of chicken, inviting the collapse of trade talks to either get the deal it wants or leave without a deal.

The EU says it cannot trust those who break agreements and that if the bill is not effectively scrapped there will be no trade deal to cover everything from car parts to food.