Both Sinn Féin and Fianna Fáil have voiced concerns over the Northern Ireland 'backstop'

The British Prime Minister has won a key vote in the House of Commons, amid ongoing political turmoil over Brexit.

Rebel Conservative MPs had put forward an amendment for Britain to remain in a customs union with the EU if no trade agreement is reached.

However, despite Labour backing the amendment, the government ultimately defeated the proposal by 307 votes to 301.

There is, however, growing concern over the impact political instability in Britain could have on Brexit negotiations.

Last night, the British government narrowly also avoided defeat by backing a series of amendments by pro-Brexit MPs.

However, significant concerns have now been voiced over the potential impact on Northern Ireland - including plans for a backstop deal to prevent a hard border.

Fianna Fáil claimed last night's votes effectively render the recent proposals for a 'soft Brexit' agreed by Mrs May's cabinet 'redundant'.

The party's Brexit spokesperson Lisa Chambers argued: "This move is the latest in a series of position changes by the British government and has cast doubt over the ability of the negotiations to deliver on a legally binding backstop."  

Sinn Féin's leader in the North Michelle O'Neill insisted the Irish government must now seek “cast iron guarantees” on the Brexit backstop.

Leo Varadkar today said that while there's likely to be "many more twists and turns over the weeks and months ahead", he suggested there "shouldn't be cause for panic" over the turmoil in Westminster.

Separately, Vote Leave - the main group that campaigned for a Leave vote in the Brexit referendum - has been fined and referred to British police for breaking electoral law.

Picture by: John Linton/PA Wire/PA Images

The British Electoral Commission found that more than £675,000 (€760,000) spent by a group called BeLeave should instead have been declared by Vote Leave.

The commission says that Vote Leave group ultimately exceeded its legal spending limit of £7 million (€7.88 million) by just under £500,000 (€563,000).