Any Republicans candidate in this year's legislative elections who makes approaches to the FN or Macron will be expelled, election campaign organiser François Baroin declared.
The former budget minister has taken over the reins of the mainstream right's campaign following the resignation of François Fillon, who failed to make it into the second round following fake-jobs allegations involving his family.
Whoever wins the 7 May presidential race will have to form a government from MPs elected in a parliamentary poll in June.
At present just two MPs support Le Pen and Macron's party has none, since it did not even exist at the time of the last parliamentary election.
Although both parties can expect to increase their representation, it is unlikely that either will have an overall majority, meaning that they must either hope that substantial numbers of sitting MPs defect to them or form a coalition with members of already-existing parties.
Baroin insisted that the Republicans will be fighting to form a government under their own colours and promised "the mother of all battles".
"Emmanuel Macron has won the battle of ambiguity," he told RTL radio. "He'll lose the battle of clarity."
And he added that any Republicans who entered an interim government between the presidential and parliamentary elections would have expelled themselves.
Socialists lose ground to left and right
The Socialists face an even stiffer challenge than the Republicans.
Faced with widespread disillusionment with President François Hollande's government, their candidate, Benoît Hamon, received only 6.36 percent of the votes.
Many of their voters went over to left-winger Jean-Luc Mélenchon, who won 19.58 percent, and his Unbowed France movement intends to field parliamentary candidates across the country.
Other former Socialist bastions have seen support drain to the FN, especially in the presidential election first round.
Many Socialist bigwigs have backed Macron in the presidential race but he has repeatedly said that to gain his movement's support Socialists or Republicans must leave their old party and join his.
Former prime minister Manuel Valls is one leading Socialist who appears to be courting Macron.
Despite having poor relations with him when they were in government, Valls supported the former economy minister against Hamon and has hinted that he would be ready to serve in a Macron government.
The presidential hopeful has said that Valls would not be a minister "because we want renewal" but has since added that En Marche ! would not stand a candidate against him if he leaves the Socialist Party.
Whether from defections or just losing seats, the parliamentary election could be a decisive blow for France's Socialist Party.