(ShareCast News) - Johnston Press shares plunged on Thursday as Numis placed its rating and target price 'under review' after the company swung to a first half loss.For the 26 weeks to 2 July, the company posted a statutory pre-tax loss of £183.7m versus a £2.2m profit the year before, as it booked a £183.6m non-cash impairment reflecting a change of assumptions on its publishing titles and print assets.Advertising revenues were down 17.9% in the first quarter but improved marginally, with a 15% drop in the second quarter despite the impact of the run-up to and uncertainty following the UK's vote to leave the European Union in June.Chief executive Ashley Highfield said the market continues to be challenging and uncertainty surrounding the outcome of the Brexit negotiations has caused further softness in some segments of the advertising market, in June and July.Numis said the half year results came in below its estimates at every level."Our key issue at Johnston remains debt which has risen to £209m (from £179.4m) representing almost 4x our annualised EBITDA."This material debt pile, relative to cashflows, in combination with the declining and uncertain trading environment place a significant question mark on the equity value in the business."Numis said it expects to cut estimates "reasonably sharply" in both fiscal years 2016 and 2017.Shares fell 17.09% to 11.40p at 1009 BST.