Derby had started strongly with Max Bird seeing a shot cleared off the line but Blackburn stunned them with three goals in the space of four minutes.

The home side were caught out by a rapid break in the 11th minute and after Ben Brereton’s shot came back off a post, Dolan tapped in the rebound.

Derby were still reeling from that when Johnson made it two a minute later with a 25-yard drive that fizzed into the bottom-left corner of Marshall’s net.

Marshall could do nothing about Johnson’s second in the 15th minute when another strike from distance took a big deflection on its way into the roof of the net.

Rovers looked like scoring every time they went forward and Brereton almost added a fourth in the 20th minute when his back-post header was saved by Marshall.

Derby’s defensive vulnerability was exposed again in the 33rd minute as Joe Rothwell surged into the box but his attempt to pass the ball into the net resulted in a simple save for Marshall.

The Scotland international rescued Derby again when Lewis Holtby played in Armstrong but Marshall made a brilliant save.

Derby had strong appeals for a penalty turned down and Louie Sibley had a header tipped over before Blackburn almost added a fourth on the break at the end of a remarkable first half.

Derby switched formation for the second half but Blackburn still created chances with Brereton firing over from a good position before Rothwell ran into the box but again failed to find the power to trouble Marshall.

Both sides were finding space and Kamil Jozwiak got in on the right in the 65th minute but could not get enough bend on the ball to find the far corner.

Derby had another chance seven minutes later when Nathan Byrne’s cross from the right was met by Lee Buchanan but he headed over from eight yards.

Any doubts about the outcome were removed in the 77th minute when Harry Chapman broke clear on the right and set up Armstrong for a close-range finish – his sixth goal of the season.