Foreign Secretary William Hague was due to hold talks on Friday with the leader of Myanmar's pro-democracy movement, Aung San Suu Kyi, a key player now that a civilian government is trying to end Western sanctions against the country.

Hague is the first foreign minister from the former colonial power to visit Myanmar since 1955, before the army took over in what was known as Burma in 1962.

The army ran the reclusive country until elections in 2010 that ushered in an army-backed civilian government last March, which has since embarked tentatively on the road to reform and opening up to the rest of the world.

Other developed countries are carefully seeking to engage with Myanmar's new rulers, keen to improve living conditions in the country but also to let their companies invest in the rich natural resources there, which are already being exploited by Asian countries such as China, Thailand and India.

Suu Kyi, who spent years in detention until just after the 2010 poll, plans to run for parliament in by-elections on April 1. The authorities formally registered her National League for Democracy as an authorised party on Thursday.

Hague met members of the former junta now running the nominally civilian administration in the capital Naypyitaw on Thursday, urging them to release all remaining political prisoners and ensure the by-elections were fair.

In a shift away from Britain's usually tough stance on Myanmar, Hague said he recognised the government seemed committed to reforms and said sanctions could be withdrawn if it released political detainees, ended human rights abuses by the army, solved ethnic conflicts and guaranteed free elections.

My message is, if you want those sanctions, those restrictive measures, lifted, then it's important to complete this process of reform. We believe now that you are sincere about it, so now, get ahead quickly and complete it, he told reporters in Naypyitaw.

Britain is the biggest aid donor to Myanmar.

SUU KYI THE KEY

U.S. Secretary of State Hillary Clinton made a landmark visit to Myanmar late last year, seeking the same reforms and offering similar concessions.

On Thursday, the European Union said it planned to open a representative office in the main city, Yangon.

Hague is in a tricky situation, likely to be criticised by vocal opponents of the junta at home if he is seen as cosying up to an old guard that may still be pulling the strings, but keen to encourage a reform movement that surprised many in the West.

Britain expressed guarded optimism after the release of 230 political prisoners last October, but just 12 political detainees were thought to have been freed this week among 900 prisoners set free as an Independence Day gesture. As many as 600 may remain behind bars.

Suu Kyi is a key player because of her influence at home and abroad. Analysts and diplomats say a decision to withdraw her long-standing support for sanctions would make it easier for Britain and others to scale down the embargoes.

She's crucial to driving this process, said a British diplomat. She has a lot of sway.

The new government may be happy to see her in parliament: Suu Kyi and her party will give the assembly more legitimacy but an army-dominated party will still control it, along with the military representatives who have a quarter of the seats.

The Nobel peace laureate gave a guarded welcome to the reform process in an interview with BBC television on Thursday.

I don't think the pace of change is as fast as a lot of us would like it to be but, on the other hand, I don't think it's too slow. It's slow but it's not too slow, she said.

Parliamentary speaker Thura Shwe Mann, a powerful member of the former junta, made it clear to Hague he wanted Suu Kyi on board. He said laws had been amended to allow her party to contest the by-elections, which would be fair, according to prepared comments seen by Reuters.

(Editing by Alan Raybould and Paul Tait)