The poll shows the most substantial Tory lead for two weeks.
Ukip was down to 11 per cent with Greens and Lib Dems trailing on seven and 11 per cent respectively.
The Tories are up two points since Ashcroft's last national poll while Labour are unchanged on 30 per cent.
Six out of the last eight Ashcroft polls have shown Tory leads.
An ICM poll for the Guardian today had the Tories ahead by three points but Populus had Labour in front by the same number. YouGov, meanwhile, puts Ed Miliband's party ahead by one point.
On the party leaders, little has changed. Around half of those polled said they preferred David Cameron to Miliband, while just under 30 per cent said they preferred the Labour leader.
Both parties have been struggling to break out of the mid-to-low 30s and have been running neck-and-neck for the entirety of the General Election campaign.
In addition to the national poll, Lord Ashcroft examined three marginals and one semi-marginal constituency.
Labour are polling behind the Tories in the marginal seat of Great Yarmouth, which is currently held by Conservative candidate Brandon Lewis, but have pulled well ahead in the seat of Great Grimsby, where Labour MP Austin Mitchell is stepping down.
Trade union officer Melanie Onn is hoping to succeed Mitchell as the MP for the area.
Labour are also ahead of Conservative hopeful Aidan Burley in the battleground of Cannock Chase, but the Tories remain favourites to win Castle Point, which at one point looked vulnerable to a Ukip insurgency.
The marginal as well as the national polls are bad news for Ukip.
Nigel Farage's party has seen its share of the vote fall in two constituencies polled. However, they have moved into the main challenger slot in Great Grimsby, replacing the Conservatives.
Voting intentions from my four latest constituency polls. Are UKIP falling back in their strong areas? pic.twitter.com/bDHqvLZFOJ— Lord Ashcroft (@LordAshcroft) April 27, 2015