The tightening in national polls which has seen the Conservatives overhaul Labour in recent surveys may not be reflected in the key marginal seats which will decide the May 7 general election, new research has found.

Locally, the polls show Labour could claim the Wirral West seat of Conservative employment minister Esther Mcvey.

Recent polling has shown the two main parties absolutely neck-and-neck as they head into the election battle, with Tories topping eight surveys to Labour's seven in the first few days of this month.

But surveys of eight marginal constituencies for former Tory treasurer Lord Ashcroft found a swing towards Labour in five over the past few months, and only two where Conservatives had improved their position.

If the results were repeated on May 7, Labour would comfortably hold one of its most vulnerable seats in Southampton Itchen - where outgoing MP John Denham had a majority of just 192 in 2010.

Ed Miliband's party would overturn Tory majorities of 2,583 in Chester, 2,969 in Croydon Central, 2,023 in Halesowen & Rowley Regis, 2,069 in Nuneaton and 2,436 in Wirral West, where it could claim the scalp of Ms McVey.

Compared to polls taken between August and December last year, the new survey shows Labour gaining 10 points in Chester, eight in Southampton Itchen, four in Wirral West, two in Nuneaton and one in Halesowen.

The Tory position had improved only in Croydon Central, where the party gained two points since October but remained behind Labour, and in Worcester, where a two-point Labour lead was transformed into a six-point Conservative advantage over the same period.

South Swindon remained a virtual dead heat, with Tories only a point ahead in a seat which they won by 3,544 votes (7.5%) in 2010.

Lord Ashcroft said: "My findings ... suggest the better national news for the Conservatives is not spread evenly across the battleground.

"Even with only just over seven weeks to go, it is important to remember that these polls are still snapshots, not predictions.

"As other polling suggests, and as we know from my focus groups, the situation is fluid, with with many undecided or in a dilemma."