With 'Hustle' also achieving 5.1 million, just 93,000 viewers separated the two shows.
'Law & Order: UK', which has two more episodes left in its run, was watched by 20.2% of the audience and 'Hustle', which finishes next week, by 19.8%.
The 9-10pm slot witnessed the launch of a new four-part Channel 4 series challenging politicians including former Tory leader Iain Duncan Smith to live with poor people.
'Tower Block of Commons' drew 1.8 million viewers and an 8.2% share, including the contribution of C4+1.
It was beaten by the final part of BBC Two's 'The British Family: Our History', which managed 1.9 million viewers and a 7.4% share.
Five was close behind with an audience of 1.6 million or 6.2% of the audience tuning in for 'Paul Merton in Europe'.
ITV1
ITV1 draws 5.1 million audience with 'Law & Order UK'
LONDON - The closely fought Monday night battle between the two big channels was edged by ITV1 last night as 'Law & Order: UK' brought in 5.1 million viewers, according to unofficial overnight figures.
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