Marks & Spencer will launch a high-profile 20 per cent off promotion on Thursday as retailers compete more aggressively for share of Christmas spend.

Debenhams began a “three-day spectacular” on Wednesday, promising more than£200 million of price cuts.

As revealed exclusively by Retail-Week.com on Tuesday, Marks & Spencer intends to retaliate with an event of its own tomorrow, offering 20 per cent off - its biggest pre-Christmas promotion for four years.

The move will potentially ignite a bloody high street trading war ahead of what is likely to be the toughest Christmas retailers have faced in a generation.

A Marks & Spencer spokeswoman declined to comment on promotional plans.

However, when he unveiled interim results earlier this month, Marks & Spencer executive chairman Sir Stuart Rose said he would watch developments on the high street closely and act appropriately to ensure the retailer was among the Christmas winners.