Unlikely relationship: Lady Thatcher is back at Chequers for private lunch with the Browns

Lady Thatcher will return to Chequers this weekend for a private lunch with Gordon Brown that will seal one of the unlikeliest relationships in British politics.

It will be only the second time that the former Prime Minister has visited the Buckinghamshire estate since she was forced out of office in 1990.

Mr Brown invited her after she expressed an interest in the house during her high-profile visit to Downing Street last year.

She will join the Premier and his family on Saturday, for what is hoped will be a chance to admire the grounds and gardens of the Elizabethan mansion in the sunshine of late summer.

Lady Thatcher developed a great fondness for Chequers during her 11 years as Premier. Friends have described how she cried as she left the house for the last time.

She has returned only once, for the unveiling of a stained glass window bearing her coat of arms in 1995.

News of her visit is likely to revive the controversy that followed her appearance at Mr Brown's side on the doorstep of No 10.

But it comes at a useful time for the Prime Minister, who is desperate to persuade voters that he remains a 'conviction politician' in the Thatcher mould, despite being at rock bottom in the opinion polls.

Last year Tories accused the Labour leader of exploiting a 'frail old woman' for political purposes, only to be slapped down by the lady herself and her closest allies, led by Lord Tebbit.

Labour MPs, for their part, expressed horror that their leader could associate himself with the party's longstanding enemy.

Mr Brown had devoted his early political career in Scotland to opposing Lady Thatcher and her policies, and his newfound enthusiasm for his predecessor was striking.

As an ambitious young MP in 1989, he had published a book entitled 'Where there is greed: Margaret Thatcher and the betrayal of Britain's future'.

He had also previously distanced himself from the praise lavished on Lady Thatcher by Tony Blair.

Lady Thatcher's visit to Downing Street came at the height of Mr Brown's triumphal first few months as Premier and fuelled speculation that he was preparing to call a general election in the autumn.

He invited her after she wrote to him to congratulate him on taking office.

The Iron Lady took tea with the family, brought toys for Mr Brown's young sons John, four, and Fraser two, and told them about her new cat.

Lady Thatcher's willingness to pose with Mr Brown last year was seen in some quarters as a deliberate dig at David Cameron, who has distanced himself from her legacy.

Lord Tebbit praised Mr Brown for his 'principled conduct', and suggested he was the real heir to Margaret Thatcher.