Gordon Brown will make his first major intervention of the general election campaign in the West Midlands, England, on Thursday 11 May.
The former prime minister is expected to deliver a speech in the Labour heartland after ex-John Lewis boss turned Conservative candidate Andy Street beat Siôn Simon to become the metro-mayor for the region on 4 May.
Details of Brown's speech – his first address since speaking on Brexit and Scotland's future in March – have not been released by Labour.
But a party source in the city where he is scheduled speak told IBTimes UK that local campaigners were on the defensive as the Tories are up to 22 points ahead of Labour in the opinion polls as Ukip voters turn to the Conservatives.
"Things are going to be extremely tight, let's put it that way," the source said. As for whether Jeremy Corbyn was coming up on the doorstep in the area, the source added: "He's coming up everywhere, isn't it?"
A number of Labour-held seats in the West Midlands with narrow majorities could turn Conservative on 8 June, including Wolverhampton South West, Birmingham Edgbaston, Walsall North and Coventry South.
Nuneaton, a seat in Warwickshire considered to be a bellwether in general elections, is expected to be retained by the Tories, according to Labour sources. Marcus Jones won the constituency in 2015 with a majority of more than 4,800 votes.