It has gone to underweight from neutral while cutting its valuation to 307p from 550p.
Referring to its bid to shore up market share by dropping prices, the broker said: “This time around M&S is operating in a significantly more competitive environment, and reducing prices by only 2%.
“Volume uplifts are therefore likely to be materially lower than in the past.”
JPMC has lopped 17% from its 2017 profit forecasts and 20% from its 2018 and ‘19 numbers.
The shares opened 1.7% lower at 350p, and are down by almost 40% in the year to date.
They are off almost 90p since last week’s profit warning, which prompted the focus on price.
Last week M&S chief executive Steve Rowe said he wanted to go back to basics with a focus on “stylish everyday essentials.”
The company outlined plans to simplify its ranges, reduce promotions, improve availability and increase the number of staff in its shops.
But Rowe’s warning that the overhaul would take time and hit short-term profits did not go down well in the City.
Clothing like-for-like sales fell 2.7% in the fourth quarter and 2.9% for the year as a whole.
Rowe, who replaced Marc Bolland earlier this year, said: "Recovering our clothing & home business won't happen overnight.
"It will take time for customers to notice the improvements we are making and change their shopping behaviour, but we are confident that our commitment to providing the right product, price and service will help return clothing & home sales to growth.
"These actions, combined with the difficult trading conditions, will have an adverse effect on profit in the short-term."
Group underlying pre-tax profit in the 53 weeks to April 2 rose 3.5% to £689.6mln on a 0.8% increase in revenue to £10.6bn.
Pre-tax profits fell 18.5% to £488.8mln after one-off items of £200.8mln.