LONDON (Reuters) - Britain's Secretary of State for Northern Ireland, Brandon Lewis, will meet the leaders of the province's political parties on Monday to encourage them to restore government and form an executive after last week's election.

       Irish nationalist party Sinn Fein won the most seats in the Northern Ireland's devolved assembly for the first time, a result it said was a "defining moment" for the British-controlled region.

       It has the right to put forward a candidate for first minister in Northern Ireland's power-sharing government, set up under a 1998 peace deal that ended three decades of sectarian conflict in the province.

       But the pro-British Democratic Unionist Party (DUP) has said it will not participate until there is a breakthrough in post-Brexit rules that impose trade barriers between the province and the rest of the United Kingdom.

       Lewis will meet Sinn Fein leader Michelle O'Neill, her DUP counterpart Jeffrey Donaldson, and the leaders of the Alliance, UUP and SDLP parties to encourage them to restore the devolved institutions, the British government said.

       "The people of Northern Ireland deserve a stable and accountable devolved government and I will continue to urge the leaders of Northern Ireland political parties to fulfil their responsibilities and form an executive as soon as possible," he said in a statement ahead of the meetings.

       British Deputy Prime Minister Dominic Raab said on Sunday the political stability of Northern Ireland was in peril until problems with the protocol governing post-Brexit trade were fixed.

       Lewis said London "would not shy away from taking further steps if necessary" to address the protocol if agreement with the European Union could not be reached.

       "However, the people of Northern Ireland need a stable and accountable government that delivers on the issues that are important to them," he said.

       (Reporting by Paul Sandle; Editing by Kirsten Donovan)