By Lindsay Dunsmuir

WASHINGTON (Reuters) -- A U.S. judge said on Wednesday congressional Republicans could move forward with parts of a lawsuit that alleges executive overreach by President Barack Obama's administration in implementing his signature healthcare law.

U.S. District Judge Rosemary Collyer, appointed by former President George W. Bush, a Republican, said the House of Representatives has standing to pursue claims that the secretaries of health and human services and of the Treasury violated the Constitution by spending funds Congress did not appropriate.

At the same time, Collyer determined they could not pursue claims that the Treasury secretary improperly amended the healthcare law, as those concerned only the implementation of a statute and not adherence to any congressional requirement.

Collyer did not rule on the merits of the claims, only on the administration's motion to dismiss the lawsuit on the issue of standing, a requirement in U.S. law whereby plaintiffs have to show they have been directly harmed.

On that issue, "the constitutional trespass alleged in this case would inflict a concrete, particular harm upon the House for which it has standing to seek redress in this court," Collyer wrote in her opinion.

The Department of Justice will appeal the court's ruling, said spokesman Patrick Rodenbush. An appeal could further delay proceedings on the merits of the claims.

White House spokeswoman Jennifer Friedman called the decision unprecedented.

"This case is just another partisan attack, this one, paid for by the taxpayers; and we believe the courts will ultimately dismiss it," she said in a statement.

House Republicans filed the lawsuit in November, saying administration officials overreached in authorizing Treasury payments to healthcare insurers and delaying the law's employer mandate.

In a statement, U.S. House Speaker John Boehner, a Republican, said the ruling showed the Obama administration's "historic overreach can be challenged by the coequal branch of government with the sole power to create or change the law."

The 2010 Affordable Care Act, the Democratic president's biggest domestic policy achievement was bolstered by a Supreme Court decision in June that upheld federal tax subsidies that helps millions of Americans afford coverage.

The case is United States House of Representatives v. Burwell et al, U.S. District Court for the District of Columbia, No 14-1967

(Reporting by Lindsay Dunsmuir; Additional reporting by Nate Raymond, Lawrence Hurleyand Roberta Rampton; Editing by Doina Chiacu, Mohammad Zargham and David Gregorio)