Gordon Brown's not a centre court player, says Alan Johnson

Gordon Brown faced fresh questions over his leadership after a top party donor said the Prime Minister faced electoral doom and a senior cabinet minister likened him to outsiders playing on the outer courts at the Wimbledon tennis championships.

Alan Johnson, the Health Secretary, made the unflattering comparison as Mr Brown endured a weekend of fresh woes.

The Prime Minister faces another bruising by-election and a test of his standing in the party's Scottish heartlands after the veteran Labour MP David Marshall announced he was standing down in Glasgow East. Mr Brown also lost a key ally when Wendy Alexander, the Labour leader in the Scottish Parliament resigned over a donations row.

A more immediate challenge is the threat by his MPs to rebel later this week over the 10p tax fiasco and plans to introduce retrospective increases in car tax.

Significant donors to Labour have now spoken out, criticising Mr Brown's leadership and warning that the party was likely to lose the next general election.

Yesterday Mr Johnson claimed that Mr Brown had enjoyed a year of achievement since moving to No 10, but acknowledged that he was not a showman. He said: "I guess in the analogy of Wimbledon, Gordon's not interested in strutting his stuff on the show courts and he's quite happy to do it on the outer courts, which makes him quite unique for a politician.

"But in terms of what he is achieving, in very difficult circumstances, I think it's been a good year."

Yesterday Sir Gerry Robinson, the former chairman of the media giant Granada who has given £70,000 to the party since 2001, said Mr Brown "is showing all the signs of not being a capable leader". He told the BBC's Andrew Marr Show that Labour was, "probably in an impossible position to come back from".

Sir Gerry said: "These crises don't happen accidentally. It's not just an issue of timing; it's an issue of either getting it right or not getting it right. And it doesn't feel right.

"In the end, politics like everything is about leadership and leadership is about a capacity to make us feel we are doing the right thing at the right time and making everybody feel good – that the country's in good shape. If you haven't got that capacity then I wouldn't back it frankly, and I don't think Gordon has that capacity. It is a very, very difficult position to come back from. In fact, probably an impossible position to come back from."

He spoke out after a string of major donors criticised Mr Brown. The businessman Sir Maurice Hatter told The Sunday Times: "I just don't think he is a prime minister."

Bill Kenwright, the theatrical producer and chairman of Everton Football Club, told the same newspaper that the leadership needed a "quantum change".

Lord Levy, Labour's chief fundraiser under Tony Blair, said yesterday that he was "almost incredulous" that Labour had finished behind the BNP in last week's Henley by-election.

He said the party had to rally round Mr Brown or consider an alternative. "If they are just lying down and saying, 'Well it's all over we don't want to give any money because Brown is all over' then surely the Labour Party ... have to seriously consider, is this the leader we want to lead this party into the next general election?"

Yesterday Labour MPs warned that the party would face a tough fight to hold Mr Marshall's Glasgow East seat, despite his 13,507 majority making it one of the safest in Scotland.

One said: "I wouldn't like to predict that one."

Another said: "We are in some considerable difficulty. The nationalists are riding high, although whether they are riding high enough is another thing. The areas we consider safest can be the most vulnerable."