By Jim Muir BBC correspondent in Tehran

Reformists in Iran have had to postpone their first-ever congress because of divisions within their ranks.

The reformists control both the presidency and the parliament, but have been able to achieve little in the face of stiff opposition from hard-line conservatives, who still hold much of the real power despite electoral unpopularity.

Conservative clerics maintain much control across Iran

The reform movement appears to be moving into a period of crisis.

There are fears that the splits that have appeared between the reformists might prove irreparable, further weakening them.

The reformists' umbrella grouping - called the Second of Khordad Front after the date in the Persian calendar when President Mohammad Khatami scored his surprise 1997 election landslide - set no new date for the congress, which was to have begun in Tehran on Thursday.

Support but little progress

The basic problem is that the tidal wave of support for reform, which induced nearly 20 different factions to hitch their star to the reformist wagon, has not resulted in significant progress, despite all the election victories.

So, long-standing differences between the groups are coming to the fore, aggravated by new differences over how best to proceed in the face of hard-line obstruction.

There is a clear division between moderate factions and more radical reformist elements.

Political tensions are high in Iran

A clerical grouping, known as the Association of Militant Clergy, and a technocratic centrist faction, the Kargozaran or "Executives of Construction", are among the elements pulling towards the centre, and away from what some regard as dangerously radical positions being adopted by other, more impatient, reformist factions.

The latter include the biggest of the reform groups, the Mosharekat (Participation) Front, which is also accused by some of the other factions of wanting to dominate the movement, a charge it denies.

One of the key differences is over how to react if two new bills, proposed by President Khatami in a last-ditch attempt to break the deadlock over reforms, are blocked by the conservatives.

Political shifts

Some reformists favour walking out of office, arguing that anything else would break the trust placed in them by the electorate.

But the more moderate groups argue that that would be irresponsible and would also leave the conservatives free to fill the vacuum.

Reforms are not welcomed by all Iranians

The fate of the two bills may take months to resolve, and it seems unlikely that the reformists will be able to hold their congress until then - if then.

In the meantime, the splits that have come to light suggest that the more moderate reformist factions may begin moving closer to moderate factions from the conservative side of the political divide, a re-configuration that has been predicted for some time by some political analysts here.

'Reforms suppressed'

"The divisions within the Second of Khordad Front may lead to its eventual collapse," said Mohammad Hashemi, a leading member of the technocratic Kargozaran faction, which is influenced by the still-powerful former President, Hashemi Rafsanjani.

President Khatami's brother Mohammad-Reza, who heads the Mosharekat faction, struck a depressed note as he opened a recent party youth meeting.

"The powerful [right-wing] political faction is trying to suppress the reform movement and divide the reformist forces," he said. "There is little reason for hope.

"The power of the reformists in the ruling structure is extremely limited. We can't do much more than voice the problems."