Sussex 103 for 0 (Haines 73*, Orr 17*) trail Middlesex 676 for 5 dec (Robson 253, Stoneman 174, White 110*, Andersson 88) by 573 runs
Sam Robson scored a career-best 253 and Robbie White his second successive century as Middlesex piled up the highest total in their history.
White was unbeaten on 110 when Middlesex declared on 676 for 5 on day two of their LV = Insurance County Championship match at Hove.
It was the sixth-biggest total Sussex have conceded in their history and highest for 21 years, but openers Tom Haines and Ali Orr enjoyed benign conditions too, taking their side to 103 without loss at stumps.
Haines passed fifty for the seventh time this season and at stumps he was unbeaten on 73, which included 11 fours, with Orr 17 not out.
Much of the day had been spent ticking off records as Middlesex, who had resumed on 400 for 2, batted Sussex out of the game.
Sussex's bowlers stuck manfully to their task, but the pitch has offered little assistance and the three wickets they did take came from misjudgements by the batters.
Robson moved to his double-hundred in the sixth over of the day with a boundary off Henry Crocombe and then accelerated, with his next 50 runs coming off 55 balls and taking him past his previous best of 231 against Warwickshire in 2013. It was a mild surprise when he tried to hit off-spinner Jack Carson through mid-wicket and gave Tom Haines, running across from mid-off, a catch. Robson had faced 356 balls and hit one six and 30 fours.
Carson picked up a second wicket before lunch when Max Holden was taken at bat-pad for one but White and Martin Andersson sentenced Sussex to more suffering in the baking heat during the afternoon, as they added 174 for the fifth wicket.
They went past Middlesex's previous highest score of 642 for 3 against Hampshire at Southampton in 1923 and just before tea White, who made his maiden century against Derbyshire at Lord's last week, took a boundary off Haines to reach his 100.
Andersson had played more fluently and looked on course for a century of his own but on 88 he tried to hit Delray Rawlins over the top and couldn't clear long on. That was four balls before tea, but Middlesex opted to finish the over after the interval before Orr and Haines could run off to put their pads on after 161 overs in the field. White's runs came from 193 deliveries with six fours.