The German government has blocked Edward Snowden from giving personal evidence in front of a parliamentary inquiry into NSA surveillance, it has emerged hours before Angela Merkel travels to Washington for a meeting with Barack Obama.

In a letter to members of a parliamentary committee obtained by Süddeutsche Zeitung, government officials say a personal invitation for the US whistleblower would "run counter to the political interests of the Federal Republic", and "put a grave and permanent strain" on US-German relations.

Opposition party members in the committee from the Left and Green party had for weeks insisted that the former NSA employee was a key witness and therefore would need to appear in person, not least because of concerns that Russia otherwise could influence his testimony.

However, the ruling Christian Democratic and Social Democratic parties, said that a written questionnaire would suffice. The disagreement led to the resignation of the CDU head of the committee this month.

Last June the German foreign ministry rejected Snowden's application for asylum because it was not submitted in person on German soil. If Snowden had been invited as a witness, he could have met these requirements.

Given that only the government could supply Snowden with permits for entering and staying in the country, as well as legal protection from an extradition query, it now looks highly unlikely that the whistleblower will be able to travel to Germany before his asylum in Russia expires at the end of June. Snowden's lawyer Jesselyn Radack said on Wednesday that she expected his Russian visa to be renewed.

Opposition politicians said they would seek ways to challenge the government's veto. The Green party leader, Simone Peter, accused the chancellor of cowardice.

"Merkel is displaying cowardice towards our ally America," she said. "We owe the Americans nothing in this respect. The government must at least make a serious effort to safely bring Snowden to Germany and let him give evidence here. But Merkel doesn't want that."

On Friday Merkel's spokesman, Steffen Seibert, said that even though Berlin last year pressed for a bilateral "no-spy" pact with Washington, "concrete results" were not expected during Merkel's US visit.

On Tuesday German government officials confirmed that Merkel would raise the issue of NSA surveillance during her scheduled four-hour meeting with Obama, but that the situation in the Ukraine and the transatlantic trade agreement (TTIP) would dominate the agenda.