England coach Trevor Bayliss accepts the Test side's top order has been their Achilles heel "for the last six or seven years" as he looks to World Cup winner Jason Roy

to make a success of the position during the Ashes.

After Andrew Strauss retired in 2012, England great Alastair Cook got through a dozen opening partners of his own before calling time on his Test career last year.

Meanwhile the vexed question of who should bat at number three has been another long-running issue during Bayliss's four-year reign, which will end after an Ashes series that starts in Birmingham next week.

Fears about England's fragility at the top of the order were reinforced by their first-innings collapse to 85 all out in the first session of their inaugural Test against Ireland at Lord's this week.

That their bowlers bailed them out of trouble by dismissing Ireland for just 38 on Friday to seal a 143-run win did little to allay the longstanding concerns over England's specialist batsmen.

Bayliss, asked if England's top order presented the team's biggest problem, replied: "You don't have to be Einstein to work that out.

"They have been for the last six or seven years, but it didn't stop us (winning the Ashes) four years ago," the Australian added.

The top three who played against Ireland -- Test debutant Roy, Rory Burns and Joe Denly -- have all been retained in a 14-man squad for the first Test against Australia that begins at Edgbaston on Thursday.

Burns averages just 22.28 in seven Tests and Denly 24.16 in three.

Now England are hoping Roy can follow the example of Australia's David Warner by taking his white-ball form into the Test arena.

Fresh from playing a key role at the top of he order in England's victorious World Cup campaign, Roy made 72 in his second Test innings against Ireland.

The 28-year-old has played the bulk of his first-class career with Surrey as a middle-order batsman and doubts remain about his ability to cope with new-ball seam movement.

"Like any debutant, he looked nervous but to score 70-odd in your first Test was a good effort," said Bayliss.

- 'Selective' -

"There was a bit more in those wickets than I'm sure he's been used to in white-ball cricket over the last few years but runs are runs. He wouldn't be the first player to look scratchy and eke out runs. In fact, that's a good sign, I think.

"We want him to go out and play his natural game but in red-ball cricket you have to be a little more selective. You've got to make a conscious effort to say to yourself, 'I'm not going to go for the big cover drive on the up until I'm really settled, the wicket is flat or the ball's not doing as much'.

"Jason probably looked a little scratchy but he got 72 and helped us win the game."

Traditionally, the best batsman in a Test side has come in at number three but England captain Joe Root, the most talented member of his side's top order, prefers to bat at number four.

But this means the Yorkshireman has started many of his recent Test innings with England in trouble having lost two cheap wickets.

"Joe knows how I feel," Bayliss said. "It's been my thought for a few years (that Root should bat at number three). "But he's the captain and he'll make the final decision."