MITCHELL Johnson undermined England's ambitions to retain the Ashes in the third Test, with a devastating spell of four wickets for seven runs at the WACA.

Johnson's intervention appeared to come in the nick of time for Australia, after England had moved serenely on the second morning to 78 without loss in reply to 268 all out.

Johnson (four for 24) broke the opening stand when Alastair Cook fell for his first score under 50 so far in the series, a push-drive sliding off the face of the bat low to gully - where Michael Hussey took a neat low catch.

After the left-armer's next over from the Prindiville Stand end, Jonathan Trott and Kevin Pietersen were also gone - for four runs between them - and England stumbled to 119 for five at lunch.

Johnson suddenly found previously elusive late swing to the new batsmen, for two lbws as both Trott and Pietersen shaped to drive.

The latter, who had made a man-of-the-match and career-best 227 in England's innings victory at Adelaide two weeks ago, went for a third-ball duck this time. Pietersen chanced a DRS review, but simulation confirmed Johnson was too good for him on this occasion.

Australia's mercurial pace bowler therefore continued to justify his recall, the native Queenslander having made a defiant half-century yesterday on what is now his home ground.

Australia, could not afford to miss any opportunities, but did so when Andrew Strauss (52) edged Ryan Harris between a static wicketkeeper and first slip for four when he had just 16. The England captain played and missed several times but went on to count eight boundaries in a 95-ball half-century.

But with Johnson in full cry, Harris got his man caught behind with a delivery which did not swing but brought a deserved outcome for the bowler's perseverance.

Johnson, meanwhile, was far from done with - and soon made Paul Collingwood his fourth victim, and third to go lbw to inswing. Umpire Marais Erasmus initially ruled the ball had not hooped in quite enough to hit the stumps. But after Johnson requested DRS evidence, Hotspot overturned the verdict.