What happens next with UK bill breaching Brexit divorce treaty?

British Prime Minister Boris Johnson is pressing ahead with legislation on trade despite a warning from Brussels that it could wreck their future relationship and an acknowledgement by his government that it violates international law.

The Internal Market Bill is aimed at ensuring Britain's four constituent nations can trade freely with one another after leaving the European Union, but the government says that requires overriding part of the withdrawal treaty it signed with Brussels.

The EU has threatened legal action against Britain, and many lawmakers have voiced concern about the prospect of breaching an international treaty.

WHAT HAPPENS NEXT?

The bill must pass through both houses of British parliament to become law, first the House of Commons, where Johnson's Conservative Party has an 80-seat majority, and then the House of Lords, the upper chamber, where it does not have a majority.

The debate will begin after 1430 GMT on Monday in the House of Commons, where the principle of the bill will be debated and lawmakers will decide whether it should go to the next stage.

If passed on Monday, there will be four more days of debate on the bill's fine print -- on Tuesday and Wednesday, and then Sept. 21 and Sept. 22. There are likely to be votes on attempts to change the wording and meaning of the law, and a final vote to decide whether it goes to the next stage.

If the bill passes the lower house, it will undergo scrutiny in the House of Lords. This has not been scheduled yet.

CAN LAWMAKERS BLOCK OR CHANGE IT IN THE HOUSE OF COMMONS?

Yes, but Johnson's big majority, won on the basis of pushing ahead with Brexit, makes this difficult.

Any attempt to block or alter the bill requires opponents to assemble a majority in parliament. This would require at least 40 Conservatives rebelling and all opposition parties uniting behind a single position.

The extent of any potential rebellion in Johnson's party is hard to gauge, but an amendment put forward by Conservative lawmaker Bob Neill is attracting some support. It seeks to give parliament a veto on any decision to breach the Withdrawal Agreement.

Labour and other opposition parties have yet to set out their own position on this and other amendments.

WHAT ABOUT THE HOUSE OF LORDS?

Many members of the upper house have criticised the bill, including Conservatives, but their primary role is to amend and improve legislation, not to block it on principle.

While there is precedent for the chamber blocking legislation, any decision to do so on this bill would provoke a constitutional row.

The House of Lords is more likely to seek to amend the bill to remove or dilute certain parts. The amendments would go back to the House of Commons for approval.

If Johnson's lower-house majority holds firm, the bill could bounce back and forth between the two chambers until either a compromise is found or the government attempts to pass it without the House of Lords' approval.

CAN THE GOVERNMENT BACK DOWN?

After the government said it was prepared to violate international law, and following the criticism it has received, it is hard to see how or why it would back down without getting a concession or concessions in talks with the EU.

The policy is described as a 'safety net' by ministers, to protect Northern Ireland's position if a deal on future relations with the EU cannot be reached. Reaching a deal with Brussels might allow the government to abandon this safety net.