The charismatic Fischer will testify before a parliamentary investigation seeking to establish how relaxing the rules for issuing visas led to tens of thousands of eastern Europeans pouring into Germany between 2000 and 2003.

The opposition Christian Democrats say the system was ruthlessly exploited by people smugglers who brought in women forced into prostitution, drug dealers, workers seeking illegal employment and even suspected terrorists.

For years Germany's most popular politician, Fischer's standing has slipped in the wake of the visa scandal.

Testimony ahead of crucial election

He will give his version of events just weeks before a key state election on May 22 in Germany's most populous state of North Rhine-Westphalia.

As a leading member of the Greens, the junior partners of Chancellor Gerhard Schröder's Social Democrats (SPD) in the ruling coalition, a weak performance from Fischer could have an impact on the state election.

With a general election on the horizon in September 2006, it would be a crushing blow to the SPD if it were to lose in a state which is traditionally one of its strongholds.

Ambassadors put pressure on Fischer

The cross-party committee of members of parliament will undoubtedly be treated to a display of Fischer's formidable speaking skills, but the question is how he will explain his failure to shut the floodgates more quickly.

Current and former German ambassadors have said in testimony that they sent alarming reports of the massive scale of the applications to Fischer's ministry from as early as March 2000.

The embassy in the Ukrainian capital Kiev for example issued 297,000 visas in 2001 alone, but the foreign ministry did not fully close the loophole until March 2003.

The former ambassador to Russia, Ernst-Jörg von Studnitz, told the commission this week that a report he sent "was on the radar screen, the question is whether he (Fischer) looked at it."

No plans to resign

Fischer, who has Schröder's firm backing, has admitted errors in his handling of the affair, but has ruled out resigning.

"I should have acted in a more energetic and comprehensive way between 2000 and 2002, that is true," he told Bild newspaper last month. "But since then we have got a grip on the problem."

Deputy questioned for 10 hours

A former deputy to Fischer at the foreign ministry, Ludger Volmer, was grilled for 10 hours by the committee on Thursday during which he denied the easing of visa requirements had led to an influx of criminals.

The basic idea of the reform had been "to solve humanitarian problems" and to reunite families separated by Europe's Cold War division, Volmer said. "The point was not to open up whole sections of the borders."

But media reports say that is effectively what happened, with hundreds of thousands of Ukrainians for example being granted visas after simply stating they wanted to visit Cologne cathedral.

Opposition ready to attack

Eckart von Klaeden, a leading member of the opposition Christian Democratic Union (CDU), predicted a rough ride for Fischer on Monday.

Von Klaeden said on Friday that testimony already heard by the investigation had shown the foreign ministry under Fischer had "for years been flying blind, guided by ideology and ignorance."

Volmer's marathon questioning was televised live, and Fischer's appearance will also be shown as it happens, a first in Germany.